We need xsetuid() and xsetgid() because per-user process resource limits can

prevent a process from switching to a user that has too many processes, and
when that happens WE'RE STILL ROOT.  See http://lwn.net/Articles/190331/

libbb/xfuncs.c

@@ -282,3 +282,15 @@ char *itoa(int n)
 	return local_buf;
 }
 #endif
+
+#ifdef L_setuid
+void xsetgid(gid_t gid)
+{
+	if (setgid(gid)) bb_error_msg_and_die("setgid");
+}
+
+void xsetuid(uid_t uid)
+{
+	if (setuid(uid)) bb_error_msg_and_die("setuid");
+}
+#endif