* libc/include/stdio.h (renameat): Declare for Cygwin.

	* libc/include/stdio.h (renameat): Declare for Cygwin.
	(symlinkat): Ditto.
	* libc/include/sys/unistd.h (readlink): Align declaration to POSIX.
	(faccessat): Declare for Cygwin.
	(fchownat): Ditto.
	(linkat): Ditto.
	(readlinkat): Ditto.
	* libc/include/sys/_default_fcntl.h (AT_FDCWD): Define for Cygwin.
	(AT_EACCESS): Ditto.
	(AT_SYMLINK_NOFOLLOW): Ditto.
	(AT_SYMLINK_FOLLOW): Ditto.
	(AT_REMOVEDIR): Ditto.
	(futimesat): Declare for Cygwin.
	(openat): Ditto.
	(unlinkat): Ditto.
	* libc/include/sys/stat.h (fchmodat): Ditto.
	(fstatat): Ditto.
	(mkdirat): Ditto.
	(mkfifoat): Ditto.
	(mknodat): Ditto.