unix_chkpwd is designed to have a suid bit, but it's not possible to set it
for files in the store.  This patch tells unix_pam.so to look for
unix_chkpwd in setuid program directory on Guix System.

--- a/modules/pam_unix/Makefile.in
+++ b/modules/pam_unix/Makefile.in
@@ -651,1 +651,1 @@
-	-DCHKPWD_HELPER=\"$(sbindir)/unix_chkpwd\" \
+	-DCHKPWD_HELPER=\"/run/setuid-programs/unix_chkpwd\" \
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<tr><td><span title='2020-03-10 20:26:52 +0100'>2020-03-10</span></td><td><a href='/guix/commit/doc/fdl-1.3.texi?id=21656ffa3b6d78a610f0befced20cc9b4b3baab6'>doc: Use HTTPS for external links.</a><span class='msg-avail'>...</span></td><td>Vitaliy Shatrov</td></tr>