We inherited from TRE regexec code that's utterly wrong with respect
to the integer types it's using; while it doesn't appear to be unsafe,
it fails to find matches past offset INT_MAX. This patch fixes the
type of all variables/fields used to store offsets in the string from
int to regoff_t, and seems to fix the problem, though it has not been
heavily tested yet.

I've also attached a test program suitable for demonstrating the bug
and at least one case where the fix works. It uses my (also attached)
alloc_huge function which allows testing >4GB inputs to string
functions without the need for huge amounts of physical memory or
swap.

Rich