MAC: mac.h: Fix missing includes/typedefs
    
    This fixes inclusion from userspace, and also kernel inclusion for
    source files not including explicitly nor implicitly <sys/types.h>.
    
    Reviewed by:    jamie
    Approved by:    markj (mentor)
    MFC after:      5 days
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D46896
    
    (cherry picked from commit 5140ccf2268b0e771c89e46f46c60e1fee7637c3)