Fix Ethernet devices unable to discover others

[OninDynamics]

On my machine connected via Ethernet, Warpinator silently breaks and discovery stops working (Shows "No other computers found..."), and running warpinator shows a KeyError in src/networkmonitor.py:

Traceback (most recent call last):

  [earlier calls omitted...]

  File "/usr/lib/warpinator/server.py", line 199, in add_service
    if not self.netmon.same_subnet(remote_ip_info):
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
  File "/usr/lib/warpinator/networkmonitor.py", line 170, in same_subnet
    self.current_ip_info.ip4["netmask"]))
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
KeyError: 'netmask'

Apparently, in networkmonitor.py, self.current_ip_info.ip4 can contain a mask key instead of netmask, breaking the device discovery functionality. Manual connections still work, though.

This PR (very crudely) fixes this by allowing either "mask" or "netmask" as the key name as discussed in my comment in #220

Do note that this is the first time I've touched Python, and I might have missed something important. It is possible the correct fix could be somewhere else (maybe how netifaces returns interface info), so feedback is appreciated.