audit(4): Add tests for the fr class of syscalls

readlink and readlinkat are the only syscalls in this class. open and
openat are as well, but they'll be handled in a different file. Also, tidy
up the copyright headers of recently added files in this area.

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15636