MEDIUM: config: implement maxsslconn in the global section

MEDIUM: config: implement maxsslconn in the global section

SSL connections take a huge amount of memory, and unfortunately openssl
does not check malloc() returns and easily segfaults when too many
connections are used.

The only solution against this is to provide a global maxsslconn setting
to reject SSL connections above the limit in order to avoid reaching
unsafe limits.