Change parameter default to always available value

`SIGTERM` is only defined in ext/pcntl, and as such never available on
Windows.  Moving the constant to ext/standard does not make much sense,
because that parameter is actually unused on Windows.  Therefore, we
use the magic number `15` instead, what is also done in the PHP manual.

Author: cmb69

ext/standard/basic_functions.stub.php

@@ -1175,7 +1175,7 @@ function proc_open($cmd, array $descriptorspec, &$pipes, ?string $cwd = null, ?a
 function proc_close($process): int {}
 
 /** @param resource $process */
-function proc_terminate($process, int $signal = SIGTERM): bool {}
+function proc_terminate($process, int $signal = 15): bool {}
 
 /** @param resource $process */
 function proc_get_status($process): array {}

ext/standard/basic_functions_arginfo.h

@@ -1805,7 +1805,7 @@ ZEND_END_ARG_INFO()
 #if defined(PHP_CAN_SUPPORT_PROC_OPEN)
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_proc_terminate, 0, 1, _IS_BOOL, 0)
 	ZEND_ARG_INFO(0, process)
-	ZEND_ARG_TYPE_INFO_WITH_DEFAULT_VALUE(0, signal, IS_LONG, 0, "SIGTERM")
+	ZEND_ARG_TYPE_INFO_WITH_DEFAULT_VALUE(0, signal, IS_LONG, 0, "15")
 ZEND_END_ARG_INFO()
 #endif