Improved NMEA Parser

[hpmax]

I put an update into my fork of variod. I'd like some people to try testing before I do a PR.

This update makes a few improvements:

1. Support is added to the NMEA extension syntax as defined by @bomilkar to query the status of some vario functionality (Polars, MacCready, Bugs, Wingloading).
2. This support is now bi-directional. Variod will query XCSoar for this information on startup and every 5-6 seconds until XCSoar provides the desired information (currently defaulting to Polar and MacCeady). Note that support for this was added in XCSoar 7, XCSoar 6.8 will ignore these requests.
3. Sentences can now be split across multiple messages.

Improvements that I'd still like to see:

1. The $POV protocol should be expanded to include the ability to query Vario/STF mode (i.e. $POV,?,VAR).
2. The $POV protocol should be expanded to include the ability to query and set volume and mute. The current protocol only allows for increase or decrease in volume and toggle mute on/off.

These changes can be easily added to my code, but until the protocol is agreed upon and implemented in XCSoar not much point in doing anything.

[bomilkar]

I'd like to point to #14 and #20 .