winbindd: track whether a result from xid2sid was coming from the cache

winbindd: track whether a result from xid2sid was coming from the cache

This is needed in preparation of moving the step to update the idmap
cache from the per-idmap-domain callback wb_xids2sids_dom_done() to the
top-level callback wb_xids2sids_done().

Currently the sequence of action is:

* check cache, if not found:
  * ask backends
  * cache result from backend
* return results

Iow, if we got something from the cache, we don't write the cache.

The next commit defers updating the cache to the top-level callback, so
the sequence becomes

* check cache, if not found:
  * ask backends
* cache results
* return results

This has two problems:

* it needlessly writes to the cache what we just got from it

* it possibly overwrites the ID_TYPE_BOTH for a SID-to-xid mapping in
  the following case:

  - existing ID_TYPE_BOTH mapping in the cache, eg:

    IDMAP/SID2XID/S-1-5-21-2180672342-2513613279-2566592647-512 -> Value: 3000000:B

  - someone calls wb_xids2sids_send() with xid.id=3000000,xid.type=ID_TYPE_GID

  - cache lookup with idmap_cache_find_gid2sid() succeeds

  - when caching results we'd call idmap_cache_set_sid2unixid() with the
    callers xid.type=ID_TYPE_GID, so idmap_cache_set_sid2unixid() will
    overwrite the SID-to-xid mapping with ID_TYPE_GID

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13802

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>