Another round in fixing getopt problems. Hopefully the last one!

Another round in fixing getopt problems.  Hopefully the last one!
This time we use the getopt stuff from gnulib and define a prefix
unconditionally so that there are no collisions with any other
getopt implementations.  This is a slight waste of space in case
we already use a GNU getopt implementation, but on
the other hand it really simplifies life.

* src/include/getopt.h: Use gnulib version.

* src/include/getopt_int.h: Use gnulib version.

* src/include/groff-getopt.h: Removed.  Obsolete now.

* src/include/lib.h: Regarding getopt, don't handle any platform
specially; just include getopt.h.
(__GETOPT_PREFIX): Define (before getopt.h).

* src/include/Makefile.sub (HDRS): Updated.

* src/libs/libgroff/getopt1.c, src/libs/libgroff/getopt.c: Use
gnulib version.

* src/libs/libgroff/Makefile.sub (EXTRA_CFLAGS): Set
__GETOPT_PREFIX.

* src/utils/pfbtops/pfbtops.c, src/utils/xtotroff/xtotroff.c
(__GETOPT_PREFIX): Define (before getopt.h).