pgsql: Introduce GUC shared_memory_size

Introduce GUC shared_memory_size

This runtime-computed GUC shows the size of the server's main shared
memory area, taking into account the amount of shared memory allocated
by extensions as this is calculated after processing
shared_preload_libraries.

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/bd1788051b02cfddcd9ef0e2fd094972f372b8fd

Modified Files
--------------
doc/src/sgml/config.sgml            | 14 ++++++++++++++
src/backend/postmaster/postmaster.c |  7 +++++++
src/backend/storage/ipc/ipci.c      | 22 ++++++++++++++++++++++
src/backend/utils/misc/guc.c        | 12 ++++++++++++
src/include/storage/ipc.h           |  1 +
5 files changed, 56 insertions(+)