Adds the -p option to zfs holds

This allows for printing a machine-readable, accurate to the second,
hold creation time in the form of a unix epoch timestamp.

Additionally, updates relevant documentation and man pages accordingly.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mohamed Tawfik <m_tawfik@aucegypt.edu>
Closes #13690
Closes #14152