protocol/ArgParser: fix integer overflow in parse_range()

Casting std::numeric_limits<unsigned>::max() to "long" leads to an
overflow if sizeof(unsigned)==sizeof(long), and the result will be -1.

This happens on some 32 bit architectures, for example ARM and WIN32.

Workaround: use std::numeric_limits<int>::max(), which is the largest
signed integer.  Since sizeof(long)>=sizeof(int), this will never
overflow.

Fixes Mantis ticket 0004080.