ndc: re-instate the command sequence num

Some prior change removed the ability to use a command sequence number.
This would cause conflicts with java-land sending commands, and results
don't get routed appropriatly: ndc would catch results destined for java.
Also, cleanup the bad errno handling: don't use errno after any library
call.

Bug: 5886205
Change-Id: I72cafb7d8f8328a6879971e6dd5401f8c20a21f9