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/basic/user-util.h

@@ -130,6 +130,7 @@ int putsgent_sane(const struct sgrp *sg, FILE *stream);
 #endif
 
 bool is_nologin_shell(const char *shell);
+const char* default_root_shell(const char *root);
 
 int is_this_me(const char *username);