dhclient: Return non-zero status when script exits due to a signal

r343896 made it such that a non-zero exit status was passed through, but was
still wrong if the script exits on a signal. POSIX does not say what the
WEXITSTATUS macro returns in this case and in practice 0 is a common value.

Instead, translate the wait status into 8 bits the same way as the shell
calculates $?.

Reviewed by:	kib, Nash Kaminski
MFC after:	1 week

sbin/dhclient/dhclient.c

@@ -2348,7 +2348,8 @@ priv_script_go(void)
 	if (ip)
 		script_flush_env(ip->client);
 
-	return WEXITSTATUS(wstatus);
+	return (WIFEXITED(wstatus) ?
+	    WEXITSTATUS(wstatus) : 128 + WTERMSIG(wstatus));
 }
 
 void