Skip privilege drop when already running as target user

When rc.d uses daemon_user to run the process as an
unprivileged user, the config may also specify the same
user.  setgroups requires root and fails with EPERM.
Skip the drop if getuid already matches the target.

Author: renaudallard

thinproxy.c

@@ -1730,6 +1730,8 @@ drop_privs(const char *user)
 		logmsg(LOG_ERR, "unknown user: %s", user);
 		return -1;
 	}
+	if (getuid() == pw->pw_uid)
+		return 0;
 	if (setgroups(1, &pw->pw_gid) == -1 ||
 	    setgid(pw->pw_gid) == -1 ||
 	    setuid(pw->pw_uid) == -1) {