new acl extension and restrictted shell in crew

[Vadim Gelfer]

i have written acl hook and restrictted shell and put them in crew.

acl hook is in hgext/acl.py.
restrictted shell is in contrib/hgsh.

with restricted shell, remote user can pull/push changes, but cannot
log in to interactive shell.

with restricted shell and acl, remote user can push changes, but not
push changes that touch files in locked down subtrees of repository
that has acls turned on.