Clear POP3 capabilities in QueryCapabilities instead of the CAPA callback handler
Clear POP3 capabilities in QueryCapabilities instead of the CAPA callback handler

This way, if the server replies with -ERR in a subsequent CAPA query,
all capabilities are cleared. If the clearing happens in the callback
handler, then they are only updated if the server replies with a +OK
response.

Fixes issue #174

jamie-dainton/MailKit,mganss/MailKit,sstraus/MailKit,matt2003/MailKit,nachocove/MailKit,jstedfast/MailKit,eino-makitalo/MailKit,dsbenghe/MailKit