In gettext-base < 1.8.2, like the one we had in Wheezy, gettext.sh
references the environment variable ZSH_VERSION, which we do not
set. This has prevented us from doing `set -u` in all gettext:ized
shell scripts unless we first initialize that variable before sourcing
gettext.sh.

Now that we install a new enough gettext-base, we can finally do this
and remove the initialization hacks.

Will-fix: #9371