Filesystem: Add support for Amazon EFS mount helper

mount.efs, the mount helper for Amazon Elastic File System (EFS)
provided by amazon-efs-utils [1], is a wrapper for mount.nfs4. It offers
a number of AWS-specific mount options and some security improvements
like encryption of data in transit.

This commit adds support by treating an fstype=efs like fstype=nfs4 for
the most part.

Resolves: RHBZ#2049319

[1] https://docs.aws.amazon.com/efs/latest/ug/efs-mount-helper.html

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>