Multi-language support fixes

* Copy keys in per-language message tables so that they don't point to
free()d memory.

* Only warn about missing/extra messages if there are any.

* In language_read_list(), do not allocate languages "." or "..".
git-archimport-id: srvx@srvx.net--2004-srvx/srvx--devo--1.3--patch-37