Add close-on-exec flag for file descriptors.

I was surprised to find that dup()'ed fds don't share the close-on-exec flag.
That means it has to be stored separately from the FileDescriptor object.

VirtualFileSystem/UnixTypes.h

@@ -25,6 +25,8 @@ namespace Unix {
 #define F_GETFL 3
 #define F_SETFL 4
 
+#define FD_CLOEXEC 1
+
 /* c_cc characters */
 #define VINTR 0
 #define VQUIT 1