pgsql: Cleanup of pg_numa.c

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(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
https://postgr.es/m/CALdSSPi5fj0a7UG7Fmw2cUD1uWuckU_e8dJ+6x-bJEokcSXzqA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3887d0cfeb3315bc4c6219bd2eb90e89cf487fa8

Modified Files
--------------
contrib/pg_buffercache/pg_buffercache_pages.c | 2 +-
src/backend/storage/ipc/shmem.c | 40 ++++++++++++++++++++++++-
src/include/port/pg_numa.h | 3 --
src/include/storage/shmem.h | 2 ++
src/port/pg_numa.c | 42 +--------------------------
5 files changed, 43 insertions(+), 46 deletions(-)