[PATCH] RFC2449 "CAPA" support

Since some clients want capabilities to be described (e.g. KMail complains 
that it can't determine UIDL support), I implemented the "CAPA" command as 
per RFC 2449. The capabilities set also includes "pass" (it's identical to 
NGPopper's).

See
http://www.acm.jhu.edu/~trisk/popa3d-1.0-capa.diff

-Trisk