getopt: merge from gnulib: function prototype adjustments

getopt: merge from gnulib: function prototype adjustments

For standards compliance, getopt, getopt_long, and getopt_long_only in
glibc have to take 'char *const *argv' even though they can mutate the
array.  gnulib has tried to clean this up as much as possible: all the
internal functions use 'char **argv', and when used standalone, so do
getopt_long and getopt_long_only.

Also brought over are __nonnull annotations, corrections to documentation,
and apparently it is no longer necessary to worry about conflicting
prototypes for getopt.  The macroification of the definitions of
getopt and __posix_getopt goes beyond what is currently in gnulib.

At this point getopt1.c and getopt_int.h are identical to their gnulib
versions.

	* posix/getopt.h: Add backup definition of __nonnull for
	consistency with gnulib.  Define __getopt_argv_const to const
	if not already defined.
	(getopt): Update doc comment from gnulib.  Prototype
	unconditionally.  Add __nonnull annotation.
	(__posix_getopt): Add __nonnull annotation.
	(getopt_long, getopt_long_only): Use __getopt_argv_const in
	prototypes for consistency with gnulib.  Add __nonnull
	annotations.
	* posix/getopt.c (_getopt_initialize, _getopt_internal_r)
	(getopt_internal): Change 'argv' argument to type 'char **'.
	Remove now-unnecessary casts.
	(getopt, __posix_getopt): Eliminate repetition with a macro.
	Cast 'argv' to 'char **' when calling _getopt_internal.
	* posix/getopt1.c (getopt_long, getopt_long_only):
	Use __getopt_argv_const for consistency with gnulib.
	Cast 'argv' to 'char **' when calling _getopt_internal.
	(_getopt_long_r, _getopt_long_only_r):
	Change 'argv' argument to type 'char **'.
	(main): Constify 'long_options'.
	* posix/getopt_int.h (getopt_internal, _getopt_internal_r)
	(_getopt_long_r, _getopt_long_only_r):
	Change 'argv' argument to type 'char **'.