pgsql: Move some code from shmem.c and shmem.h

Move some code from shmem.c and shmem.h

A little refactoring in preparation for the next commit, to make the
material changes in that commit more clear.

Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/CAExHW5vM1bneLYfg0wGeAa=52UiJ3z4vKd3AJ72X8Fw6k3KKrg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6ef9bee29310fa519b1b0f01221fcf1c41a33679

Modified Files
--------------
src/backend/postmaster/launch_backend.c | 1 +
src/backend/storage/ipc/Makefile | 1 +
src/backend/storage/ipc/ipci.c | 1 +
src/backend/storage/ipc/meson.build | 1 +
src/backend/storage/ipc/shmem.c | 119 +--------------------------
src/backend/storage/ipc/shmem_hash.c | 139 ++++++++++++++++++++++++++++++++
src/include/storage/shmem.h | 29 +++----
src/include/storage/shmem_internal.h | 41 ++++++++++
8 files changed, 194 insertions(+), 138 deletions(-)