Enable -Wconversion warnings.

Flags some suspect type conversions.

Fixed some warnings generated by this.
Don't use atoi().

The atoi() function will silently return 0 on conversion failure. That
is most likely not what you want when converting arguments.

Use sscanf() instead and error out on conversion errors.
Add -Werror.

Force all warnings to cause build to fail.
Merge branch 'tr/Wconversion'