pgsql: Move MAX_BACKENDS to procnumber.h

Move MAX_BACKENDS to procnumber.h

MAX_BACKENDS influences many things besides postmaster. I e.g. noticed that we
don't have static assertions ensuring BUF_REFCOUNT_MASK is big enough for
MAX_BACKENDS, adding them would require including postmaster.h in
buf_internals.h which doesn't seem right.

While at that, add MAX_BACKENDS_BITS, as that's useful in various places for
static assertions (to be added in subsequent commits).

Reviewed-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Discussion: https://postgr.es/m/wptizm4qt6yikgm2pt52xzyv6ycmqiutloyvypvmagn7xvqkce@d4xuv3mylpg4

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6394a3a61c0deae34ad9e94505d319cebe9e09a1

Modified Files
--------------
src/backend/storage/lmgr/lwlock.c | 2 +-
src/backend/utils/adt/xid8funcs.c | 2 +-
src/backend/utils/init/postinit.c | 1 +
src/backend/utils/misc/guc_tables.c | 1 +
src/include/postmaster/postmaster.h | 12 ------------
src/include/storage/procnumber.h | 13 +++++++++++++
6 files changed, 17 insertions(+), 14 deletions(-)