Committed / #3786

I've committed the fix. The main differences are:

1. function setLang() in common.inc.php calls setlocale() to set the proper locale for date formatting.
2. the config settings FORMAT_DATE, FORMAT_TIME and and FORMAT_TIMESTAMP have been removed. Proper format strings will have to be provided by translators as regular language strings. So each user will see the dates properly formatted according to its preferred language.
3. a new config setting FORCE_LOCALE has been added: it can be used to override the per-user setting and force the same locale for all languages. The most important use for this setting is that setting value "C" completely disable localized formatting. Useful if the underlying PHP platform does not have proper locale support.

There is a minor issue with the setlocale() call: the names of locales vary among platforms. So, in order to be as portable as possible, I used the feature of setlocale() that allows to specify more than one locale name. Each name is checked in order, until a valid one is found. For example for en-US language, the string is "en_US.utf8,en_US,enu". The first one "en_US.utf8" is the name used by most unices with utf8 support. The second one "en_US" is the same without utf8. The third one "enu" is the name used by Windows-based servers. If you know of any other names for platform we want to support, they must be added to the string. This string needs also be localised in every language tables.