Add a test for mount.GetMounts

Because mount.parseInfoFile is only available on Linux

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
FreeBSD's nullfs doesn't support file mount

FreeBSD doesn't have "bind" mount, but nullfs might be a similar feature.
However nullfs can mount only directories.

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Support FreeBSD on pkg/mount

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Merge pull request #4812 from kzys/freebsd-mount

FreeBSD support on pkg/mount