* imap.el: GSSAPI support, support kerberos 4 with Cyrus v1.6.x

* imap.el: GSSAPI support, support kerberos 4 with Cyrus v1.6.x
`imtest' too.
(imap-kerberos4-program): Renamed from `imap-imtest-program'.
(imap-gssapi-program): New variable.
(imap-streams): Add gssapi.
(imap-stream-alist): Ditto.
(imap-authenticators): Ditto.
(imap-authenticator-alist): Ditto.
(imap-kerberos4-stream-p): Rename from `imap-kerberos4s-p'.
(imap-kerberos4-open): Loop over imtest programs, support Cyrus
1.6.x `imtest' syntax.
(imap-gssapi-stream-p): New function.
(imap-gssapi-open): Ditto.
(imap-gssapi-auth-p): Ditto.
(imap-gssapi-auth): Ditto.
(imap-kerberos4-auth-p): Renamed from `imap-kerberos4a-p'.
(imap-send-command): Use buffer-local `imap-client-eol' value.