instance: add 'requirejail' attribute

instance: add 'requirejail' attribute

Since commit b44417c instance: provide error feedback if ujail binary is
missing, worrying log spam of the form "unable to find /sbin/jail ..."
may be encountered.

On systems not configured with jail capabilities the lack of jail binary
is not an error, whilst on systems with jail capabilities the warning
will be issued and the process is started outside of a jail.

This commit adds a new procd jail parameter 'mustjail' which if set
issues an error and does NOT start the process outside of a jailed
environment.

The original 'unable to find jail binary' warning is output in DEBUG
mode, thus processes started in a 'may jail' but non-jail capable
environment do not spam the log.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>