agent: Use clock or clock_gettime for calibration.
* agent/protect.c (calibrate_get_time): Use clock or clock_gettime.
For calibration, clock(3) is better than times(3) among UNIXen.
Tested on NetBSD 7.1 and FreeBSD 11.1, using QEMU.
Thanks to Damien Goutte-Gattat for the information of use of
CLOCKS_PER_SEC; The old code with times(3) is not 100% correct,
in terms of POSIX. It should have used sysconf (_SC_CLK_TCK) instead
of CLOCKS_PER_SEC. CLOCKS_PER_SEC is specifically for clock(3).
(cherry picked from commit 380bce13d94ff03c96e39ac1d834f382c5c730a1)