dssync: add clean_old_entries flag to dssync_ctx.

dssync: add clean_old_entries flag to dssync_ctx.

Initialize it to false.
And pass it down to the libnet_keytab context in
libnet_dssync_keytab.c:keytab_startup().

Unused yet.

Michael

Note: This might not be not 100% clean design to put this into the
toplevel dssync context while it is keytab specific. But then, on the
other hand, other imaginable backends might want to use this flag, too...
(This used to be commit 12e884f227e240860e49f9e41d8c1f45e10ad3be)