rtnl: ifinfmsg: flag conversion routines

```
>>> ip = IPRoute()
>>> ip.link('set', index=3, flags=['IFF_UP'], mask=1)
```

Bug-Url: https://github.com/svinota/pyroute2/issues/69
rtnl: ifinfmsg: symbolic flags improvement

Now the mask (`change`) is calculated automatically,
and to clean the flag one can provide it in a simple
'!FLAG' notation:

```
ip.link('set', index=dev, flags=['IFF_UP'])
ip.link('set', index=dev, flags=['!IFF_UP'])
```

Bug-Url: https://github.com/svinota/pyroute2/issues/69