Deprecate imap NIL constant

Part of https://wiki.php.net/rfc/deprecations_php_8_1.

Author: nikic

ext/imap/php_imap.c

@@ -491,7 +491,7 @@ PHP_MINIT_FUNCTION(imap)
 	imap_object_handlers.clone_obj = NULL;
 
 	/* lets allow NIL */
-	REGISTER_LONG_CONSTANT("NIL", NIL, CONST_PERSISTENT | CONST_CS);
+	REGISTER_LONG_CONSTANT("NIL", NIL, CONST_PERSISTENT | CONST_CS | CONST_DEPRECATED);
 
 	/* plug in our gets */
 	mail_parameters(NIL, SET_GETS, (void *) NIL);

ext/imap/tests/imap_open_error.phpt

@@ -23,7 +23,7 @@ try {
 }
 
 try {
-    imap_open('', '', '', NIL, -1);
+    imap_open('', '', '', 0, -1);
 } catch (\ValueError $e) {
     echo $e->getMessage() . \PHP_EOL;
 }

ext/imap/tests/nil_constant.phpt

@@ -0,0 +1,11 @@
+--TEST--
+NIL constant is deprecated
+--EXTENSIONS--
+imap
+--FILE--
+<?php
+var_dump(NIL);
+?>
+--EXPECTF--
+Deprecated: Constant NIL is deprecated in %s on line %d
+int(0)