Support setting user properties in a channel program

Support setting user properties in a channel program

This adds support for setting user properties in a
zfs channel program by adding 'zfs.sync.set_prop'
and 'zfs.check.set_prop' to the ZFS LUA API.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Co-authored-by: Sara Hartse <sara.hartse@delphix.com>
Contributions-by: Jason King <jason.king@joyent.com>
Signed-off-by: Sara Hartse <sara.hartse@delphix.com>
Signed-off-by: Jason King <jason.king@joyent.com>
Closes #9950