Use "gettext.h" instead of its complicated substitute.

Use "gettext.h" instead of its complicated substitute.
Include <errno.h>, for program_invocation_short_name
and program_invocation_name.
(__argp_basename) [!_LIBC]: Remove; the only use was
replaced by its body.
(__argp_short_program_name): Change condition from
!defined __argp_short_program_name to
! (defined _LIBC || HAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME),
to match argp-namefrob.h.
(__argp_failure): Don't assume strerror_r returns char *.