Reduce default shift used to calculate the max frequency for the TSC timecounter to 1, and correspondingly increase the precision of the gettimeofday(2) and related functions in the default configuration.
Reduce default shift used to calculate the max frequency for the TSC
timecounter to 1, and correspondingly increase the precision of the
gettimeofday(2) and related functions in the default configuration.

The motivation for the TSC-low timecounter, as described in the
r222866, seems to provide a workaround for the non-serializing
behaviour of the RDTSC on some Intel hardware.  Tests demonstrate that
even with the pre-shift of 8, the cross-core non-monotonicity of the
RDTSC is still observed reliably, e.g. on the Nehalems.  The r238755
and r238973 implemented the proper fix for the issue.

The pre-shift of 1 is applied to keep TSC not overflowing for the
frequency of hardclock down to 2 sec/intr.  The pre-shift is made a
tunable to allow the easy debugging of the issues users could see with
the shift being too low.

Reviewed by:	bde
MFC after:	2 weeks

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