Cygwin: processes: fix handling of native Windows processes

Since commit b5e1003722, native
Windows processes not started by Cygwin processes don't have a
Cygwin PID anymore.  This breaks ps -W and kill -f <WINPID>.

Introduce MAX_PID (65536 for now).

Cygwin processes as well as native Windows processes started
from a Cygwin process get a PID < MAX_PID.  All other native
Windows processes get a faked Cygwin PID >= MAX_PID.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/pinfo.h

@@ -211,6 +211,8 @@ private:
   DWORD status_exit (DWORD);
 };
 
+#define MAX_PID 65536
+
 #define ISSTATE(p, f)	(!!((p)->process_state & f))
 #define NOTSTATE(p, f)	(!((p)->process_state & f))