getopt: port recent getopt changes to macOS Problem reported by Harald Maier (Bug#26398). The macOS C compiler uses __nonnull for its own purposes and that clashes with glibc's __nonnull. * lib/getopt.in.h: Add comment for _GL_ARG_NONNULL snippet. * lib/getopt_cdefs.in.h (__nonnull): Remove. * lib/getopt_core.h (getopt): * lib/getopt_ext.h (getopt_long, getopt_long_only): Use _GL_ARG_NONNULL, not __nonnull. * lib/unistd.in.h: Move snippet hooks to before where the getopt .h files are included, so that _GL_ARG_NONNULL is defined in time. * modules/getopt-posix (Depends-on): Add snippet/arg-nonnull. (getopt.h): Interpolate _GL_ARG_NONNULL snippet.