Allow 'zfs holds -r' to recurse over a file system or volume to find holds

Previously, the parameters of 'zfs holds' could only be snapshots

Add -d <depth> flag to limit depth of recursion
Add -p flag to print literal values, rather than interpreted values
Add -H flag to suppress header output and use tabs rather than whitespace

Reviewed by: mahrens, smh, dteske
Approved by: bapt (mentor)
MFC after: 3 weeks
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D3994