The setuid helper of D-Bus is responsible for "service activation".
It looks for '.service' files in fixed locations, but the default locations
make no sense (see below), so replace them with /etc/dbus-1/system-services.

--- dbus-1.10.0/dbus/dbus-sysdeps-util-unix.c	2015-10-22 00:07:03.829251854 +0200
+++ dbus-1.10.0/dbus/dbus-sysdeps-util-unix.c	2015-10-22 00:07:14.893445175 +0200
@@ -1410,10 +1410,7 @@ _dbus_get_standard_system_servicedirs (D
    * be available.
    */
   static const char standard_search_path[] =
-    "/usr/local/share:"
-    "/usr/share:"
-    DBUS_DATADIR ":"
-    "/lib";
+    "/etc";
   DBusString servicedir_path;
 
   _dbus_string_init_const (&servicedir_path, standard_search_path);
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>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-09 23:33:04 +0100'>2020-01-09</span></td><td><a href='/guix/commit/HACKING?id=a7bde77d247ed0765f5378779d67b4668c5d7648'>doc: Add "Tracking Bugs and Patches" section.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-13 14:25:19 -0400'>2019-07-13</span></td><td><a href='/guix/commit/HACKING?id=06e1ecbf17fee7fe513ad2808d8175fb3565ae3e'>doc: Update keyserver URL.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2019-02-11 23:23:27 +0100'>2019-02-11</span></td><td><a href='/guix/commit/HACKING?id=6701edbfad62ba9572ffd35497df3db27b208ba5'>HACKING: Update key server.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-07-22 17:00:47 +0200'>2018-07-22</span></td><td><a href='/guix/commit/HACKING?id=5285192150c83db12136af6a123e03bb9b9dbe5f'>doc: Fix typo (missing period)</a><span class='msg-avail'>...</span></td><td>Pierre Neidhardt</td></tr>
<tr><td><span title='2017-08-19 13:42:52 +0530'>2017-08-19</span></td><td><a href='/guix/commit/HACKING?id=acabbd597678e1f0be995297bb009b6dd38ae795'>doc: Mention how to use emacs-debbugs.</a><span class='msg-avail'>...</span></td><td>Arun Isaac</td></tr>
<tr><td><span title='2017-03-27 00:13:18 +0200'>2017-03-27</span></td><td><a href='/guix/commit/HACKING?id=2fccc0d193497eb2ee3127270dea12be103f1bc2'>maint: Give command to install the pre-push hook.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-03-24 13:56:38 +0100'>2017-03-24</span></td><td><a href='/guix/commit/HACKING?id=ee7c21c7044355fc4c6bbe0906f061f6fbdfb79e'>maint: Mention the Signed-off-by line in 'HACKING'.</a><span class='msg-avail'>...</span></td><td>Clément Lassieur</td></tr>
<tr><td><span title='2017-03-15 15:19:53 +0100'>2017-03-15</span></td><td><a href='/guix/commit/HACKING?id=bc551cf32b6ccb3f8dd60b1d0d4e3c3e88cb1f8d'>maint: Mention guix-patches@gnu.org in 'HACKING'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-01-30 13:20:13 +0100'>2017-01-30</span></td><td><a href='/guix/commit/HACKING?id=8efc35a8902910ef7c09183c1d79d51498b2c10c'>maint: Fix invalid calls to 'info'.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>
<tr><td><span title='2017-01-04 16:27:20 -0500'>2017-01-04</span></td><td><a href='/guix/commit/HACKING?id=69355e1283d300a663fb639ec426f10f3feb404b'>doc: Add a Git hook that verifies signatures before pushing.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>