Allow airodump-ng to exit & get reaped

[cbrunnkvist]

Attempts to address #335.

It seems like airodump-ng was not exiting in a timely manner. One way that this manifests itself, is as the little 2 second "hickup" that occurs after the user sends Ctrl-c to stop scanning: if you crank verbosity up to at least 2 (-vv), you can clearly see that the process handling goes a little bit something like this when the scanning finishes:

 [?]  sending interrupt to PID 661927 (airodump-ng wlan0 -a -w /tmp/wifiteii2to_hi/airodump --write-interval 1 -c 1 --wps --output-format pcap,csv)

 [?]  Waited > 2.00 seconds for process to die, killing it

but the process never does exit - it is waiting for something - buffered output to be read, or something along those lines.

Seems like airodump-ng has a completely mute background-mode exactly for this purpose - so let's ensure it is used.

[l29ah]

Maybe submit the PR to a maintained fork like https://github.com/kimocoder/wifite2