Fix output of linprocfs stat entry

The Linux /proc/stat entry has grown over time

v2.5.41 <

user, nice, system, idle

v2.5.41

user, nice, system, idle, iowait, irq

v2.6.11

user, nice, system, idle, iowait, irq, softirq, steal

v2.6.24

user, nice, system, idle, iowait, irq, softirq, steal, guest

v2.6.32 >

user, nice, system, idle, iowait, irq, softirq, steal, guest, guest_nice

Some applications (e.g. nodejs) depend on the correct number of entries
and will abort otherwise.

Fix is to print the correct number of entries based on the value of
osrelease set either in sysctl or the jail settings. Change is similar
to approach used by illumos.

Reviewed by: emaste, imp (mentor)
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D15858