pgsql: Remove MaxBackends variable in favor of GetMaxBackends() functio

Remove MaxBackends variable in favor of GetMaxBackends() function.

Previously, it was really easy to write code that accessed MaxBackends
before we'd actually initialized it, especially when coding up an
extension. To make this less error-prune, introduce a new function
GetMaxBackends() which should be used to obtain the correct value.
This will ERROR if called too early. Demote the global variable to
a file-level static, so that nobody can peak at it directly.

Nathan Bossart. Idea by Andres Freund. Review by Greg Sabino Mullane,
by Michael Paquier (who had doubts about the approach), and by me.

Discussion: http://postgr.es/m/20210802224204.bckcikl45uezv5e4@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aa64f23b02924724eafbd9eadbf26d85df30a12b

Modified Files
--------------
src/backend/access/nbtree/nbtutils.c | 4 +-
src/backend/access/transam/multixact.c | 31 +++++++---
src/backend/access/transam/twophase.c | 3 +-
src/backend/commands/async.c | 11 ++--
src/backend/libpq/pqcomm.c | 3 +-
src/backend/postmaster/auxprocess.c | 2 +-
src/backend/postmaster/postmaster.c | 4 +-
src/backend/storage/ipc/dsm.c | 2 +-
src/backend/storage/ipc/procarray.c | 25 +++++---
src/backend/storage/ipc/procsignal.c | 37 +++++++-----
src/backend/storage/ipc/sinvaladt.c | 4 +-
src/backend/storage/lmgr/deadlock.c | 31 +++++-----
src/backend/storage/lmgr/lock.c | 22 ++++----
src/backend/storage/lmgr/predicate.c | 10 ++--
src/backend/storage/lmgr/proc.c | 17 +++---
src/backend/utils/activity/backend_status.c | 88 ++++++++++++++++-------------
src/backend/utils/adt/lockfuncs.c | 4 +-
src/backend/utils/init/postinit.c | 50 ++++++++++++++--
src/include/miscadmin.h | 3 +-
19 files changed, 218 insertions(+), 133 deletions(-)