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/test/test-user-util.c

@@ -347,8 +347,8 @@ static void test_get_user_creds_one(const char *id, const char *name, uid_t uid,
 }
 
 TEST(get_user_creds) {
-        test_get_user_creds_one("root", "root", 0, 0, "/root", "/bin/sh");
-        test_get_user_creds_one("0", "root", 0, 0, "/root", "/bin/sh");
+        test_get_user_creds_one("root", "root", 0, 0, "/root", DEFAULT_USER_SHELL);
+        test_get_user_creds_one("0", "root", 0, 0, "/root", DEFAULT_USER_SHELL);
         test_get_user_creds_one(NOBODY_USER_NAME, NOBODY_USER_NAME, UID_NOBODY, GID_NOBODY, "/", NOLOGIN);
         test_get_user_creds_one("65534", NOBODY_USER_NAME, UID_NOBODY, GID_NOBODY, "/", NOLOGIN);
 }