usr.sbin/gstat: add microsecond precision for disk latency

This patch makes gstat to show latency in microseconds if actual latency
is less than 1ms. It affects only "ms/r" and "ms/w" columns.

Before patch:
L(q) ops/s r/s kBps ms/r w/s kBps ms/w %busy Name

0    922      0      0    0.0    922  35809    0.0    2.8| nda0
0    928      2     34    0.1    926  35809    0.0    3.1| nda1

After patch:
L(q) ops/s r/s kBps ms/r w/s kBps ms/w %busy Name

0    496      1     31  0.156    495  16020  0.040    1.5| nda0
0    492      0      0  0.000    492  16020  0.042    1.5| nda1

Reviewed by: imp
MFC after: 3 days
Sponsored by: Postgres Professional
Differential Revision: https://reviews.freebsd.org/D41999