Make default_charset setting per user

[tensor]

It is nice to have a default_charset setting per user in the same way as Language is set per user.

However, if caching is enabled and if default_charset is changed, the whole cache for the user must be purged to accomodate for the new default_charset to be used for new message decoding laking the specific charset/encoding.

[tensor]

I am working on a patch.

[tensor]

There is a charset_selector($attrib) function and charsetselector handler registration.

Why and how are they used?

[alec]

Currently it looks not used, but it can be used for your feature request. Waitng for a patch.