Pull request from mkuoppal: avoid integer overflow when user types
digits and they're combined into a number by successively multiplying
intermediate value by 10 and adding new digit. Needed to avoid
triggering undefined behavior if the value overflows the largest
signed integer (actually long int).
This is a much more general fix than the code in the pull request,
which imposed an arbitrary limit for one aspect of tty input.
I'm not convinced that integer.h was the right place to add the new
AppendLongDigit() macro. I may not have caught all the places where
it is needed. files.c accumulates a value from digits but uses
unsigned int, so overflow won't trigger undefined behavior (although
it presumably ends up with a different value than what was intended).
options.c and coloratt.c accumulate smaller integers and have a limit
on the number of digits they'll use, so can't overflow.
Fixes#1254
05cbbc7181