make a gross exception for /dev/null. don't try to lock the directory.
avoid plugging log files.

Reference: /n/atom/patch/applied/filternolocknull
Date: Wed Mar  5 05:24:13 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/send/cat_mail.c	Wed Mar  5 05:23:45 2014
+++ /sys/src/cmd/upas/send/cat_mail.c	Wed Mar  5 05:23:45 2014
@@ -94,6 +94,11 @@
 		s_free(ss);
 		return 0;
 	}
+	/* avoid lock errors */
+	if(strcmp(s, "/dev/null") == 0){
+		s_free(ss);
+		return(0);
+	}
 	if(d = dirstat(s)){
 		isdir = d->mode&DMDIR;
 		free(d);