dns_server: Avoid ldb_dn_add_child_fmt() on untrusted input

dns_server: Avoid ldb_dn_add_child_fmt() on untrusted input

By using the new ldb_dn_add_child_val() we ensure that the user-controlled values are
not parsed as DN seperators.

Additionally, the casefold DN is obtained before the search to trigger
a full parse of the DN before being handled to the LDB search.

This is not normally required but is done here due to the nature
of the untrusted input.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13466

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>