Should use CLOCK_MONOTONIC

[Paul "LeoNerd" Evans]

In summary, see:

https://blog.habets.se/2010/09/gettimeofday-should-never-be-used-to-measure-time.html

Basically we should be using clock_gettime(CLOCK_MONOTONIC,...) rather than gettimeofday(). This needs looking into for portability reasons. Also, the fact it would be using the nsec-resolution 'struct timespec' instead of the µsec-resolution 'struct timeval'. Might need to create a whole *new* set of time-handling functions. Hmm.