Replace 'exits' by 'threadexitsall' in aan.

Reference: /n/sources/patch/applied/aan-threadexitsall
Date: Mon Apr 16 11:31:24 CES 2012
Signed-off-by: djc@9grid.fr

--- /sys/src/cmd/aan.c	Mon Apr 16 11:31:12 2012
+++ /sys/src/cmd/aan.c	Mon Apr 16 11:31:09 2012
@@ -79,7 +79,7 @@
 usage(void)
 {
 	fprint(2, "Usage: %s [-cd] [-m maxto] dialstring|netdir\n", progname);
-	exits("usage");
+	threadexitsall("usage");
 }
 
 static int