pgsql: Refactor CreateSharedMemoryAndSemaphores

Refactor CreateSharedMemoryAndSemaphores

For clarity, have separate functions for *creating* the shared memory
and semaphores at postmaster or single-user backend startup, and
for *attaching* to existing shared memory structures in EXEC_BACKEND
case. CreateSharedMemoryAndSemaphores() is now called only at
postmaster startup, and a new AttachSharedMemoryStructs() function is
called at backend startup in EXEC_BACKEND mode.

Reviewed-by: Tristan Partin, Andres Freund
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/69d903367cdea188bf865f4edd72a186cda9e689

Modified Files
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src/backend/postmaster/postmaster.c | 20 ++--
src/backend/replication/walreceiver.c | 3 +-
src/backend/storage/ipc/ipci.c | 178 +++++++++++++++++++---------------
src/backend/storage/lmgr/proc.c | 2 +-
src/include/storage/ipc.h | 3 +
5 files changed, 117 insertions(+), 89 deletions(-)