core: allow disabling system time correction if rtc returns time far in the future

core: allow disabling system time correction if rtc returns time far in the future

There might be (embedded) systems that get never updated (things like
e.g. entertainment systems of trains, for example) and where the adjustment of
the system clock (introduced by b10abe4bba61aebe4c667c412741193f11886298) would
do the wrong thing even if the difference between the systemd build time and
the rtc is 15 years or more.

This patch allows disabling the adjustment by setting
'clock-valid-range-usec-max' meson option to 0 or to a negative value.