Add -r to force a specific SSL/TLS protocol version.
Add -R to disable one or several SSL/TLS protocol versions.
Replace WANT_SSLV2_CLIENT and WANT_SSLV2_SERVER to WITH_SSLV2.
Issue: #30
Reported by: @Apollo2342
Always explicitly name which arguments are non-null, even if all
arguments are non-null. This is to avoid bugs where newly added
arguments are automatically non-null by accident, possibly leading to
optimisation errors.
This also fixes a few potential errors related to non-null arguments;
specifically it prevents the compiler optimising away a test for sni
being NULL in cachedsess_mkkey().
Issue: #14
Reported by: kythyria
