spearmint: Fix reading PAKSUMS checksums

On GNU/Linux, atoi overflows to 'correct' unsigned value.
On OS X, atoi caps at INT_MAX instead of overflowing. Wrong unsigned value.

Use strtoul, it correctly handles unsigned [long] values.
It's stored in an unsigned int, but overflow shouldn't be a problem
(pak checksums are unsigned ints).
Merge branch 'master' into sdl2