In order for sss_cache to work correctly, we must also signal the nss
responder to invalidate the hash table requests.

https://fedorahosted.org/sssd/ticket/1759

There is a timed desctructor in the nss responder that, when the
entry timeout passes, removes the netgroup from the hash table while
the netgroup is freed. This patch adds a hash delete callback so that if the
netgroup is removed from the hash table with hash_delete, its hash table
pointer will be invalidated. Later, when the entry is being freed, the
destructor won't attempt to remove it from the hash table.

Netgroups often have memberNisNetgroup entries included in them
that will never process correctly if we require fully-qualified
names on the nested lookup. This patch alters the behavior of
netgroup lookups to check *all* domains for an unqualified
netgroup name, instead of only the ones not requiring fully-
qualified names.

https://fedorahosted.org/sssd/ticket/2013

Function sysdb_getpwnam return more results than 1 and therefore sss_cmd_done
was called. Inside of function sss_cmd_done memory was freed,
but this freed memory was used in caller functions, therefore sssd crashed.

https://fedorahosted.org/sssd/ticket/1980

To make sure that e.g. the short/NetBIOS domain name is available this
patch make sure that the responders send a get_domains request to their
backends at startup the collect the domain information or read it from
the cache if the backend is offline.

For completeness I added this to all responders even if they do not need
the information at the moment.

Fixes https://fedorahosted.org/sssd/ticket/1951