darray: use unsigned int instead of size_t for array size

darray: use unsigned int instead of size_t for array size

size_t is too large; if we ever need it, that's the least of our
problems. Besides, when we roll our own (e.g. in keymap.h) it's already
unsigned int. Instead, add some emergency overflow check. So, why?

- It plays nicer with all the other uint32_t's and unsigned int's (no
  extensions, etc.).
- Reduces keymap memory usage by 5% or so as a bonus.

Signed-off-by: Ran Benita <ran234@gmail.com>