bhyve/slirp: Drop privileges before entering capability mode

When in restricted mode, the slirp-helper process enters a capsicum
sandbox, after which we cannot look up the uid for the "nobody" user.
Reverse the order.

Reported by:	kp
Fixes:	0e62ebd201 ("bhyve: Move the slirp backend out into a separate process")

usr.sbin/bhyve/slirp/slirp-helper.c

@@ -547,6 +547,11 @@ main(int argc, char **argv)
 
 	priv.slirp = slirp;
 
+	/*
+	 * Drop root privileges if we have them.
+	 */
+	drop_privs();
+
 	/*
 	 * In restricted mode, we can enter a Capsicum sandbox without losing
 	 * functionality.
@@ -554,11 +559,6 @@ main(int argc, char **argv)
 	if (restricted && caph_enter() != 0)
 		err(1, "caph_enter");
 
-	/*
-	 * Drop root privileges if we have them.
-	 */
-	drop_privs();
-
 	/*
 	 * Enter our main loop.  If bhyve goes away, we should observe a hangup
 	 * on the socket and exit.