Optimise use of gettext, and avoid ugly warnings if Locale::gettext is not available.

Optimise use of gettext, and avoid ugly warnings if Locale::gettext is not available.

The test suite was emitting a lot of ugly gettext warnings;
setting LC_ALL didn't solve the problem for all locale setups
(since ikiwiki remaps it to LANG, and ikiwiki didn't know about
the C locale).

People also seem generally annoyed by the messages when
Locale::Gettext is not installed, and I suspect will be
generally happier if it just silently doesn't localize.

The optimisation came about when I noticed that the gettext
sub was doing rather a lot of work each call just to see
if localisation is needed. We can avoid that work by caching,
and the best thing to cache is a version of the gettext sub
that does exactly the right thing.

This was slightly complicated by the locale setting,
which might need to override the original locale (or lack
thereof) after gettext has been called. So it needs to invalidate
the cache in that case. It used to do it via a global variable,
which I am happy to have also gotten rid of.