* fhandler_disk_file.cc (fhandler_disk_file::opendir): Check descriptor

	* fhandler_disk_file.cc (fhandler_disk_file::opendir): Check descriptor
	created by cygheap_fdnew constructor.
	* fhandler_virtual.cc (fhandler_virtual::opendir): Ditto.
	* fhandler_socket.cc (fhandler_socket::accept): Ditto and move
	creation of file descriptor behind blocking OS call.
	* net.cc (cygwin_socket): Ditto.
	(cygwin_rcmd): Ditto.
	(cygwin_rresvport): Ditto.
	(cygwin_rexec): Ditto.
	(socketpair): Ditto.