[libcasper] Update cap_dns API to not trigger unused variable warnings when disabled

When compiling without casper these API calls result in unused variable warnings.
Using #defines was lovely in the past but unfortunately it triggers warnings
which can cascade into errors.

Instead, just inline with some fallthrough functions and keep things happy.

Tested:

• gcc-6 targeting mips32, with casper disabled

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D26762