Various bugs

Hi, I'm working with Christian Fuchsberger on minimac.
I've discovered a couple of bugs while working with libStatGen 1.0.8 which I re-applied here.

- NewString/Grow always allocate short buffers. With towards-zero rounding, the new buffer size is always short of at least "alloc" bytes.
- Readline had an off-by-one overflow, which was fixed. I believe the current fix is saner.