Make it fully compatible with Gnus.

Make it fully compatible with Gnus.

Don't require `advice', `pces' and `path-util'.
(pop3-ssl-program-name): Use `executable-find' instead of `exec-installed-p'.
(pop3-movemail): Don't use `write-region-as-binary'.
(pop3-open-server): Don't use `open-network-stream-as-binary'.
(pop3-open-ssl-stream): Don't use `as-binary-process'.
(pop3-open-tls-stream): Ditto.
(mail-source-fetch-pop): In Gnus, advise it to bind `pop3-connection-type' and
 `pop3-leave-mail-on-server' while fetching mails, according to `mail-sources'.