mishandling multi-byte chars in DML export to cpimport

Investigating a crash in Sky, I found a large problem in our code that exports data to cpimport (LDI). In many places we're assuming chars are 1 byte, and mixing things like byte count and character count.

The function is ha_mcs_impl_write_batch_row_() in ha_mcs_dml.cpp. It'll be obvious when you look at it.