ktrdump(8): Capsicumify

We restrict the (optional) input file and output files. It would be
nice to restrict the KVM files, but that's up to libkvm.

We wait until after kvm_nlist() is invoked to cap_enter() because
kldsym() isn't supported in the Capsicum sandbox.

Feedback from: emaste@ (earlier versions)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D7921