starttls.el: Merge with my GNUTLS based starttls.el.

starttls.el: Merge with my GNUTLS based starttls.el.
(starttls-gnutls-program, starttls-use-gnutls)
(starttls-extra-arguments, starttls-process-connection-type)
(starttls-connect, starttls-failure, starttls-success): New
variables.
(starttls-program, starttls-extra-args): Doc fix.
(starttls-negotiate-gnutls, starttls-open-stream-gnutls): New
functions.
(starttls-negotiate, starttls-open-stream): Check
`starttls-use-gnutls' and pass on to corresponding *-gnutls
function if it is set.