cmd: send compressed ZFS stream by using -c flag on send

[pabloyoyoista]

Allows the usage of built-in zfs compression by using the -c flag on zfs send. It can be very useful and save time and power if the pool is already compressed.

[someone1]

Thanks for the MR! Didn't track when this was added to the zfs send options but good to know! I assume most people would be using lz4 on their streams which wouldn't compress as nicely as the default gzip option - but that won't always be the case!