When converting built-in SID to unix GID/UID a confusing debug
message about the failed conversion was printed. This patch special
cases these built-in objects.

https://fedorahosted.org/sssd/ticket/1593

https://fedorahosted.org/sssd/ticket/1612

This patch changes the handling of ghost attributes when saving the
actual user entry. Instead of always linking all groups that contained
the ghost attribute with the new user entry, the original member
attributes are now saved in the group object and the user entry is only
linked with its direct parents.

As the member attribute is compared against the originalDN of the user,
if either the originalDN or the originalMember attributes are missing,
the user object is linked with all the groups as a fallback.

The original member attributes are only saved if the LDAP schema
supports nesting.

The element being reallocated is part of the "group_attrs" array, not
attrs.

https://fedorahosted.org/sssd/ticket/1647

A logic bug in the LDAP provider causes an attempt to allocate a zero-length
array for group members while processing an empty group. The allocation
would return NULL and saving the empty group would fail.

Allocating temporary context on NULL helps vind memory leaks with
valgrind and avoid growing memory over time by allocating on a
long-lived context.

https://fedorahosted.org/sssd/ticket/1376