Fix a regression in 6.8 where the 'single' headings option was not recognized.

Taken from upstream:

  https://git.savannah.gnu.org/cgit/texinfo.git/commit/?id=13a8894fe2faa45b04033d7122a8fe7939ce6aa2

diff --git a/tp/Texinfo/XS/parsetexi/end_line.c b/tp/Texinfo/XS/parsetexi/end_line.c
index 4556780052..3fc2065616 100644
--- a/tp/Texinfo/XS/parsetexi/end_line.c
+++ b/tp/Texinfo/XS/parsetexi/end_line.c
@@ -778,8 +778,8 @@ kdbinputstyle_invalid:
     case CM_headings:
       {
         if (!strcmp (line, "off") || !strcmp (line, "on")
-            || !strcmp (line, "double") || !strcmp (line, "singleafter")
-            || !strcmp (line, "doubleafter"))
+            || !strcmp (line, "single") || !strcmp (line, "double")
+            || !strcmp (line, "singleafter") || !strcmp (line, "doubleafter"))
           {
             ADD_ARG(line);
           }
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<div class='path'>path: <a href='/guix/log/?id=cc86d5254b3f4ddf333c6b5d8026637bfa916a12'>root</a>/<a href='/guix/log/gnu?id=cc86d5254b3f4ddf333c6b5d8026637bfa916a12'>gnu</a>/<a href='/guix/log/gnu/build?id=cc86d5254b3f4ddf333c6b5d8026637bfa916a12'>build</a>/<a href='/guix/log/gnu/build/linux-container.scm?id=cc86d5254b3f4ddf333c6b5d8026637bfa916a12'>linux-container.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/build/linux-container.scm?id=cc86d5254b3f4ddf333c6b5d8026637bfa916a12&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-09-12 23:07:43 +0200'>2019-09-12</span></td><td><a href='/guix/commit/gnu/build/linux-container.scm?id=d236cd16a7173278ca69578d3e7ce4d1ce55e04c'>linux-container: "run-container" scripts shows the container's PID.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/build/linux-container.scm (call-with-container): Add
 #:process-spawned-hook and honor it.
* gnu/system/linux-container.scm (container-script)[script]:
Define 'explain' and pass it as #:process-spawned-hook'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-06 00:26:58 +0200'>2019-07-06</span></td><td><a href='/guix/commit/gnu/build/linux-container.scm?id=935e79af61f4d903e562362cdd828be807e15581'>linux-container: Mount a new /dev/pts instance in the container.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/36463&gt;.
Reported by Steffen Rytter Postas &lt;nc@scalehost.eu&gt;.

* gnu/build/linux-container.scm (mount-file-systems): When /dev/ptmx
exists on the host, explicitly mount a new instance of devpts and make
/dev/ptmx a symlink to /dev/pts/ptmx.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-06-23 19:53:03 +0200'>2019-06-23</span></td><td><a href='/guix/commit/gnu/build/linux-container.scm?id=40c369b23442ace588d8c85f62f1741c17ed65c0'>linux-container: Remove dependency on (guix utils).</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes a bug whereby derivations importing (gnu build linux-container),
such as the 'bitlbee' and 'tor' services, would depend on the
user's (guix config) file, which was pulled as a dependency of (guix
utils).  As a result, those derivations would vary from user to user.

* gnu/build/linux-container.scm (call-with-temporary-directory): New
procedure.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-04-02 18:15:37 +0200'>2019-04-02</span></td><td><a href='/guix/commit/gnu/build/linux-container.scm?id=af76c020bf19de5fe2e92f31d8b85cbd55c481de'>linux-container: Make the guest UID and GID a parameter.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/build/linux-container.scm (initialize-user-namespace): Add
 #:guest-uid and #:guest-gid parameters and honor them.
(run-container): Likewise.
(call-with-container): Likewise.
* tests/containers.scm ("call-with-container, user namespace, guest UID/GID"):
New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-01-15 23:29:33 +0100'>2018-01-15</span></td><td><a href='/guix/commit/gnu/build/linux-container.scm?id=95aa64bc48cae061effafa7675ea9d9ccbe311a4'>linux-container: Work around EBADF errors upon exit.</a><span class='msg-avail'>...<span class='msg-tooltip'>Typically 'read-pid-file/container' would fail when starting services in
containers such as BitlBee.

* gnu/build/linux-container.scm (call-with-clean-exit): Use
'primitive-_exit' instead of 'primitive-exit'.
(container-excursion*): Close OUT.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-10-11 11:12:33 +0200'>2017-10-11</span></td><td><a href='/guix/commit/gnu/build/linux-container.scm?id=1c65cca5743e9171bbd94307195f123d26c0535e'>file-systems: 'mount-file-system' now takes a &lt;file-system&gt; object.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/build/file-systems.scm (mount-file-system): Rename 'spec' to 'fs'
and assume it's a &lt;file-system&gt;.
* gnu/build/linux-boot.scm (boot-system): Assume MOUNTS is a list of
&lt;file-system&gt; and adjust accordingly.
* gnu/build/linux-container.scm (mount-file-systems): Remove
'file-system-&gt;spec' call.
* gnu/services/base.scm (file-system-shepherd-service): Add
'spec-&gt;file-system' call.  Add (gnu system file-systems) to 'modules'.
* gnu/system/linux-initrd.scm (raw-initrd): Use (gnu system
file-systems).  Add 'spec-&gt;file-system' call for #:mounts.
</span></span></td><td>Ludovic Courtès</td></tr>