jail: Implement read-only sysctl "jail.jailed"
Implement the read-only sysctl entry 'jail.jailed', which can be used to
determine if a process is running inside a jail (value is 1) or not
(value is 0).
NOTE: The current FreeBSD has such a sysctl entry called
'security.jail.jailed'. However, DragonFly BSD doesn't not have any
'security.jail.*' but only 'jail.*' sysctl entries.
Meanwhile, update /etc/rc to use this new sysctl entry to better deal
with the rc scripts with the 'nojail' keyword.
Also document this sysctl entry in the jail.8 man page.
This commit is based mostly on FreeBSD as well as the patch in bug
report #118.
Reviewed-by: dillon, mjg (Mateusz Guzik)
Bug-report: #118