porttree Code

Remove unused (actually always used) -A option.

Refactor: separate dirs and files to ignore in params in latest_mtime().

Do not check mtime of files not affecting dependences.

Extract a part of latest_mtime() as get_mtime().

Temorally add debug prints to latest_mtime() with nesessary changes.

Do not fail on uneadable cache file.

Refactor: take renew_cache() away from main(); change name and param order in latest_mtime()

New port is not cached.

Add timestamping ports in cache and avoid rebuilding the cache, see ticket #7

Change defaults for style and for cache filename; skip base category as too special; small changes.