Allow passing a pointer to GetNamedDSMSegment()'s init callback.

This commit adds a new "void *arg" parameter to
GetNamedDSMSegment() that is passed to the initialization callback
function.  This is useful for reusing an initialization callback
function for multiple DSM segments.

Author: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAN4CZFMjh8TrT9ZhWgjVTzBDkYZi2a84BnZ8bM%2BfLPuq7Cirzg%40mail.gmail.com

contrib/pg_prewarm/autoprewarm.c

@@ -858,7 +858,7 @@ autoprewarm_dump_now(PG_FUNCTION_ARGS)
 }
 
 static void
-apw_init_state(void *ptr)
+apw_init_state(void *ptr, void *arg)
 {
 	AutoPrewarmSharedState *state = (AutoPrewarmSharedState *) ptr;
 
@@ -880,7 +880,7 @@ apw_init_shmem(void)
 	apw_state = GetNamedDSMSegment("autoprewarm",
 								   sizeof(AutoPrewarmSharedState),
 								   apw_init_state,
-								   &found);
+								   &found, NULL);
 
 	return found;
 }