Make the default handler a bit more magical... REJECT files already received

this session, SKIP files that already exist, and ACCEPT all others.

No longer try to append extra data packets to the end of the file to allow
files to grow in transit... the protocol is really not well suited to this
anyway.  Instead, truncate to the advertised length.

If the want_callback changes the file name, still ACK the correct file.

Escape filenames more consistantly.