ipfilter: Restrict ipfilter within a jail

Add a sysctl/tunable (net.inet.ipf.jail_allowed) to control whether a
jail can manage its own ipfilter rules, pools, and settings. A jail's
control over its own ipfilter rules and settings may not be desireable.
The default is jail access to ipfilter is denied.

The host system can stil manage a jail's rules by attaching the rules,
using the on keyword, limiting the rule to the jail's interface. Or
the sysctl/tunable can be enabled to allow a jail control over its own
ipfilter rules and settings.

Implementation note: Rather than store the jail_allowed variable,
referenced by sysctl(9), in a global area, storing the variable in the
ipfilter softc is consistent with ipfilter's use of its softc.

Discussed with:		emaste, jrm
MFC after:		1 week
Differential revision:	https://reviews.freebsd.org/D53623

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