$default_charset variable equal to user's locale when decoding messages.

set_my_locale function gets charset from locale if no language variable
found in user's prefs. Fixes possible problem when $default_charset and
$squirrelmail_default_language are incorrectly set in config.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@4396 7612ce4b-ef26-0410-bec9-ea0150e637f0