pgsql: Track which shmem areas have been fully initialized

Track which shmem areas have been fully initialized

If SHMEM_CALLBACKS_ALLOW_AFTER_STARTUP is used to allocate shared
memory after startup, but the initialization fails half-way through,
the shmem area is left in an indeterminate state. Furthermore, if
multiple shmem areas are registered in one RegisterShmemCallbacks()
call, some might be allocated while others are not.

This commit adds an explicit 'initialized' flag to each shmem area.
We still leave behind an uninitialized area on error, but at least
they are now clearly marked, and you get a slightly nicer error
message if you try to re-register them. It'd be nice to clean up more
thoroughly and support actually retrying the allocations, but in
practice, the most likely reason for a shmem allocation or
initialization to fail is that you are out of shared memory and
retrying wouldn't help with that.

This isn't exactly a new problem, the old ShmemInitStruct() interface
had similar issues if the initialization code failed, or if you
allocated multiple structs and some allocations failed. It was just
left to the calling code to deal with it.

Author: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/CAJTYsWVRRWH48=PcuAo_2Y4Ap6M0QRmzxgUfFkNRtdWK74LjBQ@mail.gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9d888cd3f34c4e9a8f7fbe7e78c70bbd84a481de

Modified Files
--------------
doc/src/sgml/xfunc.sgml | 5 +-
src/backend/storage/ipc/shmem.c | 67 ++++++++++++++++++++--
src/test/modules/test_shmem/Makefile | 3 +
src/test/modules/test_shmem/meson.build | 3 +
.../modules/test_shmem/t/001_late_shmem_alloc.pl | 50 ++++++++++++++--
src/test/modules/test_shmem/test_shmem.c | 3 +
6 files changed, 120 insertions(+), 11 deletions(-)