src: fix calculation of fuzzing time in statistics

When the computer is suspended during a fuzzing session,
the time spent in suspended state is counted as a "run time"
on a statistics screen.

The time returned by `gettimeofday(2)` is affected by discontinuous
jumps in the system time. It is better using `clock_gettime(2)`.

The patch replace `gettimeofday` with `clock_gettime` [1].
`clock_gettime` uses a CLOCK_MONOTONIC_COARSE clock type,
it is faster than CLOCK_MONOTONIC, but still has resolution (~1ms)
that is adequate for our purposes. However, CLOCK_MONOTONIC_COARSE
is a Linux-specific clock variant, so on macOS it is replaced
with CLOCK_MONOTONIC, and with CLOCK_MONOTONIC_FAST on FreeBSD [2].

Closes #1241

1. https://pubs.opengroup.org/onlinepubs/9699919799/functions/clock_gettime.html
2. https://man.freebsd.org/cgi/man.cgi?query=clock_gettime

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