Add TLS session support to ftp(1).

Add TLS session support to ftp(1).

If a session file is specified via the `-S session=...', ftp(1) will
attempt to resume TLS sessions based on the session data contained within
this file. Upon completion of a successful TLS handshake the session file
will be updated with new session data, if available.

Discussed with deraadt@ and beck@.

Requested by and input from espie@.