Also remove sysdb_store_domuser()

The code would call sysdb_attrs_add_uint32 which added another UID or GID
to the ID=0 we already downloaded from LDAP (0 is the default value) when
ID-mapping an entry. This led to funky behaviour later on when we wanted
to process the ID.

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

initialized variable, was causing build warning

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.

https://fedorahosted.org/sssd/ticket/1482

When we add fullname to user_attrs, then sysdb_add_basic_user()
will set fullname to gecos when it initially creates the user
object in the cache, but it will be overwritten in the same
transaction when sysdb_store_user() adds all the user_attrs.

Removing bad examples of usage of sysdb_transaction_start/commit/end
functions and making it more consistent (all files except of
src/db/sysdb_*.c).

This function is no longer necessary because we don't have fake user
entries any more. The original purpose of this function was to check if
there are fake user entries for particular user and, if yes, to update
its membership.

This function will also auto-create a new ID map if the domain has
not been seen previously.