Another messy little function for the file index btree set of btree
leaf functions has landed, leaf_split:

http://tux3.org/tux3?f=0fed5998fbec;file=test/leaf.c

This appears to work. It was able to split the sample leaf at any
entry position, include zero (leaving an empty leaf in the original
btree block) or num_entries (leaving an empty leaf in the split block).

This was not a whole lot easier to write than leaf_insert. In fact,
this whole two level design makes the coding quite hard, but...