Rename inherit_default_acl() to assign_default_acl().

The inherit_default_acl() function was called with two path names, and
the default ACL of the second path was retrieved and applied to the
first path. However, the only situation in which the function was used
was when the default ACL of the parent path was already available -- so
we were wasting time re-retrieving it.

This commit changes the name of the function to assign_default_acl(),
and it now takes an acl_t as its second parameter rather than a
path. The one place it is used now passes it the (already-known)
parent's default ACL rather than that parent's path.