pyzfs: add missing libzfs_core functions

This change adds the following libzfs_core functions to pyzfs:
lzc_remap, lzc_pool_checkpoint, lzc_pool_checkpoint_discard

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7793
Closes #7800