tools/netconsole: make a bit more robust

The netcat utility likes to exit when it receives an empty packet (as it
thinks this means EOF).  This can easily occur when working with command
line editing as this behavior will be triggered when using backspace.  Or
with tabs and command line completion.  So create two netcat processes -
one to only listen (and put it into a loop), and one to do the sending.
Once the user quits the transmitting netcat, the listening one will be
killed automatically.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>