user-util: tighten shell validation a tiny bit

src/basic/user-util.h

@@ -108,15 +108,7 @@ bool valid_user_group_name(const char *u, ValidUserFlags flags);
 bool valid_gecos(const char *d);
 char* mangle_gecos(const char *d);
 bool valid_home(const char *p);
-
-static inline bool valid_shell(const char *p) {
-        /* We have the same requirements, so just piggy-back on the home check.
-         *
-         * Let's ignore /etc/shells because this is only applicable to real and
-         * not system users. It is also incompatible with the idea of empty /etc.
-         */
-        return valid_home(p);
-}
+bool valid_shell(const char *p);
 
 int maybe_setgroups(size_t size, const gid_t *list);