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

src/include/miscadmin.h

@@ -463,12 +463,14 @@ extern PGDLLIMPORT AuxProcType MyAuxProcType;
  *****************************************************************************/
 
 /* in utils/init/postinit.c */
+/* flags for InitPostgres() */
+#define INIT_PG_LOAD_SESSION_LIBS		0x0001
+#define INIT_PG_OVERRIDE_ALLOW_CONNS	0x0002
 extern void pg_split_opts(char **argv, int *argcp, const char *optstr);
 extern void InitializeMaxBackends(void);
 extern void InitPostgres(const char *in_dbname, Oid dboid,
 						 const char *username, Oid useroid,
-						 bool load_session_libraries,
-						 bool override_allow_connections,
+						 bits32 flags,
 						 char *out_dbname);
 extern void BaseInit(void);