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

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

Make use of the _(Str) macro provided by <config.h>.

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

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