lib: consistently use _(Str) instead of gettext(Str)

lib: consistently use _(Str) instead of gettext(Str)

eu-config.h defines _(Str) to dgettext ("elfutils", Str) instead of
a simple gettext (Str) for a reason: the library might be indirectly
used by clients that called bindtextdomain with a domain different
from "elfutils".

The change was made automatically using the following command:
$ git grep -l '\<gettext *(' lib |xargs sed -i 's/\<gettext *(/_(/g'

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