Cleanup of pg_numa.c

This moves/renames some of the functions defined in pg_numa.c:

* pg_numa_get_pagesize() is renamed to pg_get_shmem_pagesize(), and
  moved to src/backend/storage/ipc/shmem.c. The new name better reflects
  that the page size is not related to NUMA, and it's specifically about
  the page size used for the main shared memory segment.

* move pg_numa_available() to src/backend/storage/ipc/shmem.c, i.e. into
  the backend (which more appropriate for functions callable from SQL).
  While at it, improve the comment to explain what page size it returns.

* remove unnecessary includes from src/port/pg_numa.c, adding
  unnecessary dependencies (src/port should be suitable for frontent).
  These were either leftovers or unnecessary thanks to the other changes
  in this commit.

This eliminates unnecessary dependencies on backend symbols, which we
don't want in src/port.

Reported-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
https://postgr.es/m/CALdSSPi5fj0a7UG7Fmw2cUD1uWuckU_e8dJ+6x-bJEokcSXzqA@mail.gmail.com

src/include/storage/shmem.h

@@ -41,6 +41,8 @@ extern void *ShmemInitStruct(const char *name, Size size, bool *foundPtr);
 extern Size add_size(Size s1, Size s2);
 extern Size mul_size(Size s1, Size s2);
 
+extern PGDLLIMPORT Size pg_get_shmem_pagesize(void);
+
 /* ipci.c */
 extern void RequestAddinShmemSpace(Size size);