glapi: Move src/mesa/main/dispatch.c to glapi and rename.

main/dispatch.c is a glapi source file.  It is part of GLAPI_SOURCES in
sources.mak and part of glapi_sources in SConscript.  This commit moves
it to glapi/ and renames it to glapi_dispatch.c.

docs/devinfo.html

@@ -107,7 +107,7 @@ Global variables are not allowed.
 Function name examples:
 </p>
 <pre>
-	glFooBar()       - a public GL entry point (in dispatch.c)
+	glFooBar()       - a public GL entry point (in glapi_dispatch.c)
 	_mesa_FooBar()   - the internal immediate mode function
 	save_FooBar()    - retained mode (display list) function in dlist.c
 	foo_bar()        - a static (private) function