libjail: start refactoring struct ioctl support

Instead of ad-hoc comparisons against the struct type in a few places,
start to abstract out an interface for dealing with struct types. For
now, this just means that we have some special jailparam_import and
jailparam_export handling for the ip addr types, but in the next commit
we'll extend it further to support MAC labels.

Reviewed by: jamie
Differential Revision: https://reviews.freebsd.org/D53959