libjail: start refactoring struct ioctl support

[kevans]

Summary

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.

[jamie]

The original author should have done this in the first place ;-)

lib/libjail/jail.c

83

You're optimistic about our coding speed :-)

[olce]

Don't have time to review this in detail, but overall you have my blessing as well. :-)