pop3: Fixed APOP being determined by CAPA response rather than by timestamp

This commit replaces that of 9f260b5d6610f3 because according to RFC-2449,
section 6, there is no APOP capability "...even though APOP is an
optional command in [POP3].  Clients discover server support of APOP by
the presence in the greeting banner of an initial challenge enclosed in
angle brackets."