Merge from gnulib

Merge from gnulib

This merges some getopt fixes from Zack Weinberg, and affects only
non-GNUish platforms.  It incorporates:
2017-04-06 getopt-gnu: omit some duplicate code
2017-04-06 getopt-posix: use angle-bracket include
2017-04-06 getopt: annotate files with relationship to glibc
2017-04-06 getopt: split up getopt.in.h and eliminate __need_getopt
2017-04-06 getopt: better handling of ambiguous options
2017-04-06 getopt: refactor long-option handling
2017-04-06 getopt: tidy up _getopt_initialize a bit
2017-04-06 getopt: merge from glibc: repetition reduction
2017-04-06 getopt: clean up error reporting
2017-04-06 getopt: fix fencepost error in ambiguous-W-option handling
2017-04-06 getopt: clean up getopt.c and getopt1.c file headers
2017-04-06 getopt: harmonize comments with glibc
2017-04-06 getopt: remove USE_NONOPTION_FLAGS
2017-04-06 getopt: tabify, in preparation for merge with glibc
2017-04-06 md5, sha1, sha256, sha512: Add comments re correctness
* build-aux/config.sub, doc/misc/texinfo.tex, lib/getopt.c:
* lib/getopt.in.h, lib/getopt1.c, lib/getopt_int.h, lib/md5.c:
* lib/md5.h, lib/sha1.c, lib/sha1.h, lib/sha256.c, lib/sha256.h:
* lib/sha512.c, lib/sha512.h, lib/unistd.in.h, m4/getopt.m4:
Copy from gnulib.
* lib/getopt_cdefs.in.h, lib/getopt_core.h, lib/getopt_ext.h:
* lib/getopt_pfx_core.h, lib/getopt_pfx_ext.h:
New files, taken from gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4:
Regenerate.