wrap userlink building with event.

Implements an event which can modify the link below usernames, and the
displayed user name.

When no name supplied, the name of currently logged-in user is used.
update $username as well, when read from _SERVER
change default arg value of userinfo in null instead false
handle interwiki without slashes as pageids. Added user interwiki

* allowed urlparams
 * added `wiki:users:` as default user profile link
wikilink needs wiki target as well
Extend showuseras config with username_link

uses the user interwiki link as profile link
added tests for Doku_Renderer::_resolveInterWiki
wikilink creating refactored to _resolveinterwiki().

Added DOKU_BASE for local target
code reformatting
Fix double slash in coral interwikilink
Use DOKU_BASE in resolveinterwiki unittest
interwiki : prefixed configurls handled as wikilinks
fix comment in _resolveInterWiki
generalised link colour in header, making interwiki links blue as well (to fit design)
change default userspace to :user:<username> and add interwiki class
update unittest with interwiki.conf change as well
added user.png for interwiki links
Merge remote-tracking branch 'origin/master' into userlink

Conflicts:
	inc/parser/renderer.php
	inc/template.php
update  usage in userlink
declare  more clear, before used as ref