Add initial set of tests for audit(4)

This change includes the framework for testing the auditability of various
syscalls, and includes changes for the first 12. The tests will start
auditd(8) if needed, though they'll be much faster if it's already running.
The syscalls tested in this commit include mkdir(2), mkdirat(2), mknod(2),
mknodat(2), mkfifo(2), mkfifoat(2), link(2), linkat(2), symlink(2),
symlinkat(2), rename(2), and renameat(2).

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15286