Removed unneeded auth check in the access API.

Some functions on the Access API (access_get_global_level,
access_get_project_level and access_has_bug_level) require an
authenticated user in order to return correct values, FALSE otherwise.

However, these functions can be used by plugins while not
authenticated, so the code was changed to allow the execution to
proceed if existing parameter $p_user_id is provided.

Fixes 0013538

Signed-off-by: Damien Regad damien.regad@merckgroup.com

Original patch was modified to follow MantisBT coding guidelines and
improve the commit message