rename settings: ssl -> tls

Currently, the manual has 15 variables starting with "ssl_": http://mutt.org/doc/manual/

The term "SSL" should be considered legacy, since it only refers to deprecated protocols: RFC 6176 and RFC 7568.

We should be using the correct term "TLS".

I propose to rename all such variables and change it in the documentation, but also allow the old settings for backward compatibility reasons.