Rename SetSingleFuncCall() to InitMaterializedSRF()

Per discussion, the existing routine name able to initialize a SRF
function with materialize mode is unpopular, so rename it.  Equally, the
flags of this function are renamed, as of:
- SRF_SINGLE_USE_EXPECTED -> MAT_SRF_USE_EXPECTED_DESC
- SRF_SINGLE_BLESS -> MAT_SRF_BLESS
The previous function and flags introduced in 9e98583 are kept around
for compatibility purposes, so as any extension code already compiled
with v15 continues to work as-is.  The declarations introduced here for
compatibility will be removed from HEAD in a follow-up commit.

The new names have been suggested by Andres Freund and Melanie
Plageman.

Discussion: https://postgr.es/m/20221013194820.ciktb2sbbpw7cljm@awork3.anarazel.de
Backpatch-through: 15

src/backend/replication/logical/launcher.c

@@ -930,7 +930,7 @@ pg_stat_get_subscription(PG_FUNCTION_ARGS)
 	int			i;
 	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 
-	SetSingleFuncCall(fcinfo, 0);
+	InitMaterializedSRF(fcinfo, 0);
 
 	/* Make sure we get consistent view of the workers. */
 	LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);