access_get_status_threshold() returns incorrect value for NEW

When the user's access level is below $g_update_bug_status_threshold and
the status to change to is NEW, the function returned the incorrect
access level, preventing user from accessing the target status when
updating bugs, even though the workflow permits it.

This commit fixes the problem by introducing special handling for NEW
status ('bug_submit_status'), in which case the function returns
'report_bug_threshold' otherwise it falls back to default
'update_bug_status_threshold'.

Fixes 0015260, affects issue 0015258