PBCKP-2705: Adjust signal handling and compatibility for PostgreSQL 18

This patch adds compatibility changes for PostgreSQL 18:

- Updated signal handler initialization in pgut.c to use sigaction()
  instead of pqsignal(), as pqsignal() now returns void.
- Updated process_block_change() in backup.c to handle the new
  RelPathStr structure returned by relpathperm() in v18.
- Updated CreateReplicationSlot_compat() in stream.c to support the
  new CreateReplicationSlot() signature introduced in v18.

These changes ensure successful build and runtime behavior with
PostgreSQL 18 while preserving backward compatibility with earlier
versions.

Author: dlepikhova

src/backup.c

@@ -2518,11 +2518,16 @@ process_block_change(ForkNumber forknum, RelFileNode rnode, BlockNumber blkno)
 	int			segno;
 	pgFile	  **file_item;
 	pgFile		f;
+#if PG_VERSION_NUM >= 180000
+	RelPathStr rel_path_str = relpathperm(rnode, forknum);
+	rel_path = rel_path_str.str;
+#else
+	rel_path = relpathperm(rnode, forknum);
+#endif
 
 	segno = blkno / RELSEG_SIZE;
 	blkno_inseg = blkno % RELSEG_SIZE;
 
-	rel_path = relpathperm(rnode, forknum);
 	if (segno > 0)
 		f.rel_path = psprintf("%s.%u", rel_path, segno);
 	else

src/stream.c

@@ -188,7 +188,10 @@ CreateReplicationSlot_compat(PGconn *conn, const char *slot_name, const char *pl
                                           bool is_temporary, bool is_physical,
                                           bool slot_exists_ok)
 {
-#if PG_VERSION_NUM >= 150000
+#if PG_VERSION_NUM >= 180000
+	return CreateReplicationSlot(conn, slot_name, plugin, is_temporary, is_physical,
+		/* reserve_wal = */ true, slot_exists_ok, /* two_phase = */ false, /* failover = */ false);
+#elif PG_VERSION_NUM >= 150000
 	return CreateReplicationSlot(conn, slot_name, plugin, is_temporary, is_physical,
 		/* reserve_wal = */ true, slot_exists_ok, /* two_phase = */ false);
 #elif PG_VERSION_NUM >= 110000