Support other FTPS PROT command options

[kevanb]

Per RFC-2228 supports four arguments to PROT:

C - Clear -neither integrity nor privacy
S - Safe - integrity without privacy
E - Confidential - privacy without integrity
P - Private - integrity and privacy

As of release 2.2.8c of FileZilla, if you specify a
remote site as server type "FTP over TLS (explicit
encryption)" (or "over SSL") the client sends "PROT P",
specifying to encrypt both control and data
connections. There should be an option in the client
to choose the level of protection you desire. Some FTP
servers (such as WS_FTP Server) now support allowing a
clear command channel after authentication for
compatibility with FTP application gateways/proxies
found in firewalls and NAT devices.