Allow clients to run "got send -d" against gotd. Clients will send a zero-id as new id for a reference and, in the special but more common case of just deleting and not updating, no pack file will be sent. ok and tweaks by stsp@

Export parse_command (now got_serve_parse_command) from lib/serve.c and use it to validate the command line in gotsh instead of just checking that the -c argument starts with 'git-receive-pack' or 'git-upload-pack'. Invalid usage now always fails before opening the socket, while before it wasn't always the case. This also means that invalid usage doesn't count towards the limits. ok jamsek, stsp

ok jamsek

ok jamsek

ok op@

ok op, jamsek

ok stsp@

ok op@