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.

fs.h

@@ -20,6 +20,7 @@ struct recursor {
 enum {
 	SAMEDEV  = 1 << 0,
 	DIRFIRST = 1 << 1,
+	SILENT   = 1 << 2,
 };
 
 extern int cp_aflag;