handle case where argument is of form /file.  e.g. aux/stub /test.
also sync usage with man page.
also add missing exits.

Reference: /n/sources/patch/applied/stub-rootfile-usage
Date: Tue Oct 19 16:30:30 CES 2004

--- /sys/src/cmd/aux/stub.c	Tue Oct 19 16:30:31 2004
+++ /sys/src/cmd/aux/stub.c	Tue Oct 19 16:30:30 2004
@@ -152,7 +152,7 @@
 void
 usage(void)
 {
-	fprint(2, "usage: aux/stub file\n");
+	fprint(2, "usage: aux/stub [-Dd] path/name\n");
 	exits("usage");
 }
 
@@ -181,10 +181,14 @@
 	if((p = strrchr(argv[0], '/')) == 0){
 		mtpt = ".";
 		kidname = argv[0];
+	}else if(p == argv[0]){
+		mtpt = "/";
+		kidname = argv[0]+1;
 	}else{
 		mtpt = argv[0];
 		*p++ = '\0';
 		kidname = p;
 	}
 	postmountsrv(&fs, nil, mtpt, MBEFORE);
+	exits(nil);
 }