ftpserver: Reworked CAPA support to allow for specifying the capabilities

Renamed SUPPORTCAPA to CAPA and added support for specifying a list of
supported capabilities to return to the client.

Additionally added the directive to the FILEFORMAT document.
ftpserver: Reworked AUTH support to allow for specifying the mechanisms

Renamed SUPPORTAUTH to AUTH and added support for specifying a list of
supported SASL mechanisms to return to the client.

Additionally added the directive to the FILEFORMAT document.