proper error handling for fcntl F_GETOWN on modern kernels

on old kernels, there's no way to detect errors; we must assume
negative syscall return values are pgrp ids. but if the F_GETOWN_EX
fcntl works, we can get a reliable answer.

arch/i386/bits/fcntl.h

@@ -31,3 +31,6 @@
 #define F_GETLK 12
 #define F_SETLK 13
 #define F_SETLKW 14
+
+#define F_SETOWN_EX 15
+#define F_GETOWN_EX 16