ACL administrators support

[Alexey M. <al...@hs...>]

I've implemented support for ACL administrators, a very simple one:

when there is a file /repos/CVSROOT/acl-admins, it should contain a
list of username who are allowed to manage ACLs in this repository.

There is no "ACL administrator can't access files as regular user"
security check yet. 

Enjoy,

--alexm