Add the CLOCK_BOOTTIME clockid for use with clock_gettime(2)

Add the CLOCK_BOOTTIME clockid for use with clock_gettime(2)
and put it to use in userspace in lieu of the kern.boottime
sysctl.

Its absolute value is the time that has elapsed since the
system booted, i.e., the system uptime.

Use in top(1), w(1), and snmpd(8) eliminates a race with
settimeofday(2), adjtime(2), etc. inherent to deriving the
system uptime via the kern.boottime sysctl.

Product of a great deal of discussion/revision with jca@, tb@,
and guenther@.

ok tb@ jca@ guenther@ dlg@ mlarkin@ tom@