common: Respect gpgconf.ctl when looking up translations

* common/i18n.c (i18n_init): Use gnupg_localedir() instead of LOCALEDIR.
(i18n_localegettext): Ditto.
* tools/gpgconf-comp.c (my_dgettext): Ditto.

On Unix, gnupg_localedir() returns the locale directory relative to
the root directory of the gnupg installation if specified in the
gpgconf.ctl. Otherwise, it returns the built-in LOCALEDIR.

• GnuPG-bug-id: T5999