MAC: Define a common 'mac' node for MAC's jail parameters
    
    To be used by MAC/do.
    
    Reviewed by:    jamie
    Approved by:    markj (mentor)
    MFC after:      5 days
    Relnotes:       yes
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D46899
    
    (cherry picked from commit 5041b20503dbb442cc9ebd0a6e4db26905102c72)