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.

test/test-execute/exec-specifier.service

@@ -26,7 +26,7 @@ ExecStart=sh -c 'test %U = $$(id -u)'
 ExecStart=sh -c 'test %g = $$(id -gn)'
 ExecStart=sh -c 'test %G = $$(id -g)'
 ExecStart=test %h = /root
-ExecStart=sh -c 'test %s = /bin/sh'
+ExecStart=sh -c 'test -x %s'
 ExecStart=sh -c 'test %m = $$(cat /etc/machine-id)'
 ExecStart=sh -c 'test %b = $$(cat /proc/sys/kernel/random/boot_id | sed -e 's/-//g')'
 ExecStart=sh -c 'test %H = $$(uname -n)'

test/test-execute/exec-specifier@.service

@@ -23,7 +23,7 @@ ExecStart=sh -c 'test %U = $$(id -u)'
 ExecStart=sh -c 'test %g = $$(id -gn)'
 ExecStart=sh -c 'test %G = $$(id -g)'
 ExecStart=test %h = /root
-ExecStart=sh -c 'test %s = /bin/sh'
+ExecStart=sh -c 'test -x %s'
 ExecStart=sh -c 'test %m = $$(cat /etc/machine-id)'
 ExecStart=sh -c 'test %b = $$(cat /proc/sys/kernel/random/boot_id | sed -e 's/-//g')'
 ExecStart=sh -c 'test %H = $$(uname -n)'