netmap: monitor: add a flag to distinguish packet direction

The netmap monitor intercepts any TX/RX packets on the monitored
port. However, before this change there was no way to tell
whether an intercepted packet was being transmitted or received
on the monitored port.
A TXMON flag in the netmap slot has been added for this purpose.

(cherry picked from commit 660a47cb991d5a7ca69cd8dd9c09a5288d49e405)