Introduce a means to conditionally wrap APIs and demonstrate it by condi...

...tionally wrapping the SSLv2 methods.

This adds `C_CUSTOMIZATION` to the various existing definition types.  It is intended to contain arbitrary C to do extra tricky things.  This is primarily useful to support maybe-available features of OpenSSL.  This branch uses it to check for SSLv2 support and offers it in the opentls API if it is found in the OpenSSL interface.

It also adds `OVERRIDES` to allow the Python definition of safe versions of the unavailable APIs to avoid tricking applications into crashing.