tls.el: New file.

tls.el: New file.
nnimap.el (nnimap-server-port, nnimap-stream): Say TLS/SSL
instead of SSL.
(nnimap-stream): Add other streams, link to imap variables.
(nnimap-authenticator): Add other authenticator, link to imap
variables.
imap.el: Autoload open-tls-stream.
(imap-streams): Add tls in front of ssl.
(imap-stream-alist): Add tls.
(imap-default-tls-port): New variable.
(imap-tls-p, imap-tls-open): New functions.