Use capsicum_helpers(3) that allow us to simplify the code and its functions
will return success when the kernel is built without support of
the capability mode.

It is important to note, that I'm taking a more conservative approach
with these changes and it will be done in small steps.

Reviewed by: jhb
MFC after: 6 weeks
Differential Revision: https://reviews.freebsd.org/D18744