various: try to use DEFAULT_USER_SHELL for root too

/bin/sh as a shell is punishing. There is no good reason to make
the occasional root login unpleasant.

Since /bin/sh is usually /bin/bash in compat mode, i.e. if one is
available, the other will be too, /bin/bash is almost as good as a default.
But to avoid a regression in the situation where /bin/bash (or
DEFAULT_USER_SHELL) is not installed, we check with access() and fall back
to /bin/sh. This should make this change in behaviour less risky.

(FWIW, e.g. Fedora/RHEL use /bin/bash as default for root.)

This is a follow-up of sorts for 53350c7bba,
which added the default-user-shell option, but most likely with the idea
of using /bin/bash less ;)

Fixes #24369.

man/systemd.unit.xml

@@ -2118,7 +2118,7 @@ Note that this setting is <emphasis>not</emphasis> influenced by the <varname>Us
           <row>
             <entry><literal>%s</literal></entry>
             <entry>User shell</entry>
-            <entry>This is the shell of the user running the service manager instance. In case of the system manager this resolves to <literal>/bin/sh</literal>.</entry>
+            <entry>This is the shell of the user running the service manager instance.</entry>
           </row>
           <row>
             <entry><literal>%S</literal></entry>