Make _bt_insertonpg() more like _bt_split().

Make _bt_insertonpg() more like _bt_split().

It seems like a good idea for nbtree's retail insert code to be
absolutely consistent with nbtree's page split code for anything that
naturally requires equivalent handling.  Anything that concerns
inserting newitem (which is handled as part of the page split atomic
action when a page split is required) should work in exactly the same
way.  With that in mind, make _bt_insertonpg() handle 'cbuf' in a way
that matches _bt_split().