pgsql: Move the shared memory size calculation to its own function

Move the shared memory size calculation to its own function

This change refactors the shared memory size calculation in
CreateSharedMemoryAndSemaphores() to its own function.  This is intended
for use in a future change related to the setup of huge pages and shared
memory with some GUCs, while useful on its own for extensions.

Author: Nathan Bossart
Discussion: https://postgr.es/m/F2772387-CE0F-46BF-B5F1-CC55516EB885@amazon.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0bd305ee1d427ef29f5fa4fa20567e3b3f5ff792

Modified Files
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src/backend/storage/ipc/ipci.c | 142 ++++++++++++++++++++++++-----------------
src/include/storage/ipc.h      |   1 +
2 files changed, 84 insertions(+), 59 deletions(-)