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
Show a domain dropdown when multiple AD domains are configured

This integrates the functionality of the now outdated addomain plugin
directly into the authad plugin and makes multi-domain setups usable
without SSO.

See also https://github.com/cosmocode/dokuwiki-plugin-addomain
change default arg value of userinfo in null instead false
add missing '&' before 't' in url construction
make media links in rss feeds absolute

This resolves issues with broken images when the wiki feed is
hosted by third parties (e.g. FeedBurner), as mentioned in
FS#2931 comments.
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
use http_sendfile correct
FS#2388 give relative path to sendfile on nginx
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
Only use the whole text as search snippet when it's the first match
Fix the snippet search to continue after the previous match
Add FULLTEXT_PHRASE_MATCH event for allowing plugins to match phrases

Our index doesn't support phrase searches so we are searching for the
pages that contain all words of the phrase and then search again in the
content of the pages. As plugins can also add additional text to the
index this event allows plugins to do phrase matching in their content.