save zone configuration as text

save zone configuration as text

as previously mentioned in commit c65b2868ab, a cfg_obj_t
configuration tree structure takes up considerably more space than
the canonical text. since the zone configuration saved in the zone
object using dns_zone_setcfg() is only currently used for "rndc
showzone", it can be saved as text more efficiently than as an
object tree. (and, if a tree were needed, the text could be
re-parsed quickly; zone configuration text is generally small.)