Warn/err on conflicting options -tls --tls-on-connect

[ametzler]

Hello,

if -tls is specified together with --tls-on-connect swaks will try to do STARTTLS on top of TLS. I suspect this is an oversight and not useful functionality (double-tunnel?) and swaks should throw a warning/err out.

This was reported as https://bugs.debian.org/1082187 - It not a straightforward user-error, the reporter had -tls enabled in .swaksrc and specified --tls-on-connect as a command-line option and got a surprising error message.

cu Andreas