Use the kern.boottime sysctl for retrieving the system boot time as a

Use the kern.boottime sysctl for retrieving the system boot time as a
struct timeval.  This is used to fix the STARTED column for pure kernel
threads.

Note, currently, we do not set the start time for pure threads due to
safety issues with microtime(9), although this change conditionalizes
the thread start time appropriately, so that in the future when pure
threads will account their start time, we wouldn't need to change anything
in this utility (with regard to the start time, ofcourse).

Discussed-with: 	Matthew Dillon <dillon@apollo.backplane.com>