r22713: Offline logon fixes for idmap manager:

r22713: Offline logon fixes for idmap manager:

(a) Ignore the negative cache when the domain is offline
(b) don't delete expired entries from the cache as these
    can be used when offline (same model as thw wcache entries)
(c) Delay idmap backend initialization when offline
    as the backend routines will not be called until we go
    online anyways.  This prevents idmap_init() from failing
    when a backend's init() function fails becuase of lack of
    network connectivity