This replaces the _glapi_tls_{Dispatch,Context} symbols with a single
symbol, _glapi_tls_Current, which references an array of type (void*).
This allows us to easily extend mapi to accomodate additional TLS
entries, which will be necessary for storing current API state and
allowing vendors to utilize TLS entries for their own purposes.

This change is rather messy. It requires modification to both the
dispatch stub generation code and mapi's compatibility #defines in
u_current.h. This could probably be cleaned up in later changes. It's
not clear whether _glapi_{set,get}_{dispatch,context}() are even needed
anymore, but they are kept in place for backwards compatibility and to
keep this change as self-contained as possible.