sysctl: Add flags to filter jail prison and vnet variables

So users do not have to contact the source code to tell whether a
variable is a jail prison / vnet one or not.

Reviewed by: cy (previous version), markj, jamie (for Jails)
MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D47107

(cherry picked from commit 5ec83c660acaf30c1d6b9417dbd8c80dfa9d56ac)