ability to enforce tls usage

[FelixSchwarz]

By default the SMTP client should enforce usage of TLS to prevent MITM downgrade attacks. It must be possible to disable TLS usage though.

CLI options from msmtp:

--tls[=(on|off)]             enable/disable TLS encryption
--tls-starttls[=(on|off)]    enable/disable STARTTLS for TLS
--tls-trust-file=[file]      set/unset trust file for TLS
--tls-crl-file=[file]        set/unset revocation file for TLS
--tls-fingerprint=[f]        set/unset trusted certificate fingerprint for TLS
--tls-certcheck[=(on|off)]   enable/disable server certificate checks for TLS
--tls-key-file=[file]        set/unset private key file for TLS
--tls-cert-file=[file]       set/unset private cert file for TLS
--tls-priorities=[prios]     set/unset TLS priorities.
--tls-host-override=[host]   set/unset override for TLS host verification.
--tls-min-dh-prime-bits=[b]  set/unset minimum bit size of DH prime