Rewritten smtpmail.el to use `open-network-stream' to do STARTTLS

Rewritten smtpmail.el to use `open-network-stream' to do STARTTLS
upgrades opportunistically, and to only use auth-source for all
credentials.  Mostly backwards compatible, but
`smtpmail-auth-credentials' and `smtpmail-starttls-credentials' are
removed, and users who relied on those will have to put the
credentials in ~/.authinfo instead.