getopt: merge straightforward changes from gnulib

getopt: merge straightforward changes from gnulib

This covers changes with little or no consequences when the code is
used in glibc.

	* posix/getopt_int.h: Include getopt.h.
	Use impl-namespace names for all arguments to _getopt_internal and
	_getopt_internal_r.
	Declare __ordering enum outside the struct.
	Harmonize comments with gnulib.
	* posix/getopt1.c: Simplify #ifdeffage at top of file. Remove
	ELIDE_CODE logic entirely.  Move inclusion of stdlib.h to
	#ifdef TEST block and make unconditional.  Do not define NULL.
	* posix/getopt.c: Partial merge from gnulib, covering the
	initial includes and global declarations, commentary, and
	a couple of semantically-neutral code changes.