MEDIUM: ssl: Set verify 'required' as global default for servers side.

If no CA file specified on a server line, the config parser will show an error.

Adds an cmdline option '-dV' to re-set verify 'none' as global default on
servers side (previous behavior).

Also adds 'ssl-server-verify' global statement to set global default to
'none' or 'required'.

WARNING: this changes the default verify mode from "none" to "required" on
the server side, and it *will* break insecure setups.