pgsql: Refactor InitPostgres() to use bitwise option flags

Refactor InitPostgres() to use bitwise option flags

InitPostgres() has been using a set of boolean arguments to control its
behavior, and a patch under discussion was aiming at expanding it with a
third one.  In preparation for expanding this area, this commit switches
all the current boolean arguments of this routine to a single bits32
argument instead.  Two values are currently supported for the flags:
- INIT_PG_LOAD_SESSION_LIBS to load [session|local]_preload_libraries at
startup.
- INIT_PG_OVERRIDE_ALLOW_CONNS to allow connection to a database even if
it has !datallowconn.  This is used by bgworkers.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/ZSTn66_BXRZCeaqS@paquier.xyz

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/4800a5dfb4c46d22b5d05f16c615bea6ff24a2bb

Modified Files
--------------
src/backend/bootstrap/bootstrap.c   |  2 +-
src/backend/postmaster/autovacuum.c |  5 ++---
src/backend/postmaster/postmaster.c | 16 ++++++++++++----
src/backend/tcop/postgres.c         |  5 +++--
src/backend/utils/init/postinit.c   | 17 +++++++++--------
src/include/miscadmin.h             |  6 ++++--
6 files changed, 31 insertions(+), 20 deletions(-)