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.

src/firstboot/firstboot.c

@@ -755,7 +755,7 @@ static int write_root_passwd(const char *passwd_path, const char *password, cons
                         .pw_gid = 0,
                         .pw_gecos = (char *) "Super User",
                         .pw_dir = (char *) "/root",
-                        .pw_shell = (char *) (shell ?: "/bin/sh"),
+                        .pw_shell = (char *) (shell ?: default_root_shell(arg_root)),
                 };
 
                 if (errno != ENOENT)