Revert "We do not have _tolower, so call tolower instead."

This reverts commit af4cdda3d8e0832e929f8098a62c3d37eb586b17.

We do have _tolower, it was just missing from the ctype.h header. Sorry
for the inconvenience!
Add _tolower() and _touppwer prototypes to <ctype.h>.

They were dropped from the header by accident.