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.

Fixes: 
 * [FS#1558](https://bugs.dokuwiki.org/index.php?do=details&task_id=1558) - Link to user page in function tpl_userinfo
 * [FS#2151](https://bugs.dokuwiki.org/index.php?do=details&task_id=2151) - more options for showuseras -- actually the suggested fix is using interwiki links...

2 question left:
 * Howto combine current behavior and a new option for own userlink
 * Howto easily configure the userprofile link?

Ideas:
 * Add 4th 'showuseras' select option: 'username with interwikilink'
 * FS#2151 suggests interwiki links for configurable links 

What is good default interwiki link? 
something like? `user      ./doku.php?id=wiki:users:{URL}`

   * should be for url-rewriting enabled imho something like `./wiki:users:{URL}`...
   * does interwiki link cover all url bases?
   * or should [FS#2713](https://bugs.dokuwiki.org/index.php?do=details&task_id=2713) be implemented firstly?

Note: `<bdi>` is added bit randomly...