Clean up proc_open() implementation

Closes GH-5507.

ext/standard/proc_open.h

@@ -17,9 +17,11 @@
 #ifdef PHP_WIN32
 typedef HANDLE php_file_descriptor_t;
 typedef DWORD php_process_id_t;
+# define PHP_INVALID_FD INVALID_HANDLE_VALUE
 #else
 typedef int php_file_descriptor_t;
 typedef pid_t php_process_id_t;
+# define PHP_INVALID_FD (-1)
 #endif
 
 /* Environment block under win32 is a NUL terminated sequence of NUL terminated

ext/standard/tests/general_functions/proc_open_redirect.phpt

@@ -53,7 +53,7 @@ proc_close($proc);
 ?>
 --EXPECTF--
 Missing redirection target
-Redirection target must be an integer
+Redirection target must be of type int, string given
 
 Warning: proc_open(): Redirection target 42 not found in %s