When invoked as "sdo_write c3 os_command command ..." this patch fixes
two thing:
- ... can now have spaces in it
- command is defined as DOMAIN which we didn't handle. There's some
  amount of stuff built around the assumption that we don't use DOMIAIN
  at all but this is what we shipped so we've got to support it now.
  Amusingly it looks like pycanopen's `encode_raw()` has a bug where it
  doesn't convert strings to bytes correctly so it's done manually here.

cfdp-py has updated with handy bug fixes! Unfortunately those conflict
with our fixes so until we can properly ship a new version to space
let's fix the version to what we've been developing with.

Similar to cmd_shell and ping, this makes it easy to coordinate with
those other scripts.

In 5f961ed I added DOMAIN handling so that we could send os commands,
but DOMAIN really is supposed to be for file-like things. This enhances
sdo_write to interpret the final argument instead as a filename, which
it then opens and sends.

Additionally os commands really should have been of type OCTET_STRING
but that needs to be fixed in oresat-configs. This adds OCTET_STRING
handling in anticipation of that.