meson: make nologin path build time configurable

Some distros install nologin as /usr/sbin/nologin, others as
/sbin/nologin.
Since we can't really on merged-usr everywhere (where the path wouldn't
matter), make the path build time configurable via -Dnologin-path=.

Closes #13028

src/sysusers/sysusers.c

@@ -361,7 +361,7 @@ static int rename_and_apply_smack(const char *temp_path, const char *dest_path)
 }
 
 static const char* default_shell(uid_t uid) {
-        return uid == 0 ? "/bin/sh" : "/sbin/nologin";
+        return uid == 0 ? "/bin/sh" : NOLOGIN;
 }
 
 static int write_temporary_passwd(const char *passwd_path, FILE **tmpfile, char **tmpfile_path) {