Enhance the current user link in the header.

When configured with SSO (using the [Reverse Proxy Auth Plugin](http://wiki.jenkins-ci.org/display/JENKINS/Reverse+Proxy+Auth+Plugin)) and IIS, Jenkins will create the hyperlink in the top-right corner as `http://example.com/user/DOMAIN/userName` which will give a 404.  The correct hyperlink, in this case, is `http://example.com/user/DOMAIN_userName`.

This pull request fixes the link's target and also will display the user's full name, if it's configured, falling back to the user ID otherwise.