ClearCase: Handle Derived Objects

Derived Objects (DO) should not be checked in or out. Ideally they should
not be edited at all, but in the case where a user by accident opens
a DO and it is read only do not offer to vcsOpen() the file.

Change-Id: I5aee9655ca8696c6d7291950946052261526c751
Reviewed-by: Orgad Shaneh <orgads@gmail.com>