Add SILENT flag to recurse()

recurse() is getting smarter every day. I expect it to pass the Turing
test in a few months.
Along the way, it was reported that "rm -f" on nonexistant files reports
their missing as an internal recurse()-error.
So recurse() knows when to shut up, I added the SILENT flag to fix all
these things.

rm.c

@@ -16,7 +16,7 @@ main(int argc, char *argv[])
 
 	ARGBEGIN {
 	case 'f':
-		rm_fflag = 1;
+		r.flags |= SILENT;
 		break;
 	case 'R':
 	case 'r':
@@ -27,7 +27,7 @@ main(int argc, char *argv[])
 	} ARGEND;
 
 	if (!argc) {
-		if (!rm_fflag)
+		if (!(r.flags & SILENT))
 			usage();
 		else
 			return 0;