Don't strip TTY device name to the last '/'.

We've seen this bug in other applications before: we have some
applications that use strrchr(tty, '/') on the TTY device name. This
isn't valid when using pts(4), because the device name will be stripped
to "0" instead of "pts/0".

This fixes issues with login(1) ignoring /etc/ttys and missing utmp
records.

Reported by:	Barney Cordoba <barney_cordoba yahoo com>
Reviewed by:	rwatson

usr.bin/login/login.c

@@ -245,8 +245,8 @@ main(int argc, char *argv[])
 		(void)snprintf(tname, sizeof(tname), "%s??", _PATH_TTY);
 		ttyn = tname;
 	}
-	if ((tty = strrchr(ttyn, '/')) != NULL)
-		++tty;
+	if (strncmp(ttyn, _PATH_DEV, sizeof _PATH_DEV - 1) == 0)
+		tty = ttyn + sizeof _PATH_DEV - 1;
 	else
 		tty = ttyn;