Add a glvnd_key_t key to GLdispatch.c to hold the current API state.

__glDispatchGetCurrentAPIState and __glDispatchGetCurrentContext are now normal
exported functions instead of wrappers around _glapi_get_current.

Removed _glapi_get_current, _glapi_set_current, _glapi_get_context,
_glapi_set_context, and their associated functions in u_current.c.

Removed the U_CURRENT_* constants and replaced them with the corresponding
GLAPI_CURRENT_* constants.

Removed the GLAPI_CURRENT_* enums except for GLAPI_CURRENT_DISPATCH.