[io-layer] Cope with pids > 16 bits

Fixes an issue in Process where "pseudo-handles" for processes with
pids > 16 bits would overflow and start getting treated as normal
handles. This would frequently segfault MonoDevelop on MacOS.

mono/io-layer/process-private.h

@@ -17,12 +17,8 @@
 #define _WAPI_PROCESS_CURRENT (gpointer)0xFFFFFFFF
 
 /* This marks a system process that we don't have a handle on */
-/* FIXME: cope with pids > 16bit */
-#if defined(__HAIKU__)
+/* FIXME: cope with pids > 31bit? */
 #define _WAPI_PROCESS_UNHANDLED_PID_MASK 0x7FFFFFFF
-#else
-#define _WAPI_PROCESS_UNHANDLED_PID_MASK 0x7FFF
-#endif
 #define _WAPI_PROCESS_UNHANDLED (-1 & ~_WAPI_PROCESS_UNHANDLED_PID_MASK)
 
 extern gpointer _wapi_process_duplicate (void);