Consistently define _(Str) using dgettext ("elfutils", Str)

Consistently define _(Str) using dgettext ("elfutils", Str)

Move the definition of _(Str) macro to lib/eu-config.h which already
provides a definition of N_(Str) macro.  Since lib/eu-config.h is
appended to config.h, it is included into every compilation unit
and therefore both macros are now universally available.

Remove all other definitions of N_(Str) and _(Str) macros from other files
to avoid conflicts and redundancies.

The next step is to replace all uses of gettext(Str) with _(Str).

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>