This implements the notion of a "vendor ID" that is unique to each
vendor library instance loaded in a process.  When entrypoint rewriting
is implemented, this will be used by libglvnd to determine whether a
given entrypoint implementation is compatible with a given context, by
comparing the vendor owning the context with the vendor owning the
implementation.

This changes the function prototype of __glDispatchMakeCurrent()
to include the vendorID as an argument, which will be saved off in the
API state, and return a boolean value to indicate whether make current
succeeded.