Add support for zfs mount -R <filesystem>

This commit adds support for mounting a dataset along with all of
it's children with '-R' flag for zfs mount. There can be scenarios
where we want to mount all datasets under one hierarchy instead of
mounting all datasets present on system with '-a' flag.

'-R' flag should work on all root and non-root datasets. Usage
information and man page has been updated for zfs mount. A test
for verifying the behavior for '-R' flag is also added.

Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #16015