Implement experimental hook-like system, with working example.

Implement experimental hook-like system, with working example.

This system allows you to call functions that may or may not exist at runtime
and compile-time. You create hook_t pointers and register them during
initialization, and point them to a function. If they're not registered,
no problem. This could potentially allow different subsystems to function
independently of each other, such as the client and server.

It's using a union to allow functions to have any return type they want.
However, for obvious reasons, functions cannot have any parameters they want
and must accept a pointer to the union type where the actual args are stored.

This is experimental and there may be bugs, but the current working example
should be stable.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12603 d7cf8633-e32d-0410-b094-e92efae38249