fmping:  Factor out protocol_{in,out} constants
fmping:  Handle ping frames

Respond to pings (with pongs) so fmping can be its own server.
fmping:  Use unet-connect

Connect upstream via the unet metaprotocol.  If no connector is set, use
uloop (loopback) as the default.
Merge branch 'fmping-unet'