slattach: pass up exit status of shell command

This is partially to fix a build warning:
	slattach.c: In function 'main':
	slattach.c:771:9: warning: ignoring return value of 'system',
		declared with attribute warn_unused_result [-Wunused-result]

And partially because I think it's the right thing to do.  Otherwise
there's no way for people to check the exit status of the command they
told slattach to run.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

slattach.c

@@ -768,7 +768,7 @@ main(int argc, char *argv[])
 
 	tty_close();
 	if(extcmd)	/* external command on exit */
-		system(extcmd);
+		exit(system(extcmd));
   }
   exit(0);
 }