Introduce caspermocks.

The idea behinds mocks is that we don't need to ifdef a lot of code in
tools itself but those defines are hidden in the casper library.
Right now the mocks are implemented as define/inlines functions.
There was a very long discussion how this should be implemented.
This approach has some advantages like we don't need to link to any additional
libraries. Unfortunately there are also some disadvantages for example it is
easy to get library out of sync between two versions of functions or that we
need extra define to compile program with casper support.
This isn't an ideal solution but it's good enough for now and should simplify
capsicumizing programs. This also doesn't close us any other ways to do those
mocks and this should evolve in time.

Discussed with: pjd, emaste, ed, rwatson, bapt, cem, bdrewery
Differential Revision: https://reviews.freebsd.org/D8753