Add $wgLocalInterwikis to handle multiple local prefixes

This is akin to $wgSkipSkin/$wgSkipSkins. It is quite plausible for a wiki
to have more than one self prefix (e.g. enwiki has w: en: wikipedia: and
maybe others).

Some recent changes code seems to use $wgLocalInterwiki for quite unclear
purposes:
- I removed the line using $wgLocalInterwiki from the RecentChange
  class, as the 'lang' field of $mExtra is not used anywhere in core code.
  Extensions may use it, but it would seem more appropriate for them to
  use something like $wgDBname (or indeed to consult $wgLocalInterwikis
  directly) if they need to identify a particular wiki.
- In the IRC formatter, the first prefix in the array is used (if set).
  Appropriate documentation is added to DefaultSettings.php.

Related to bug 954 comment 3.

Bug: 954
Bug: 955
Change-Id: I9dbb566385b464402c5e78510b95dd2ffb4d9489