Revise work with struct timeval to not rely on signedness of tv_sec/tv_usec #1937
Description
The https://www.gnu.org/software/libc/manual/html_node/Calculating-Elapsed-Time.html document suggests that there may be systems which account struct timeval fields tv_sec/tv_usec as unsigned values, so under-flows are possible when subtracting times from each other. The sample method from that document was adapted as difftimeval() in #1936; possibly the original method is also desirable to replace our loops currently similar to this:
tv.tv_usec -= difftimeval(now, start);
while (tv.tv_usec < 0) {
tv.tv_sec--;
tv.tv_usec = 1000000 + tv.tv_usec; // Note it is negative
}