mapi: Add mapi and share the code with glapi.

Specifically, move all or most of

      glapi/glapi.c         to mapi/u_current.c,
      glapi/glapi_execmem.c to mapi/u_execmem.c,
      glapi/glthread.[ch]   to mapi/u_thread.[ch]

and remove their dependencies on core Mesa headers.
19 files changed