B"H view1090: handle disconnection

 - keep trying to reconnect to server if disconnected
B"H view1090: do not block

 - do not block on network data
B"H view1090: sleep a bit between loops

view1090 was using close to 100% CPU before, with the non-blocking
commits and the reconnection code. sleep a bit between loop iterations
to keep CPU usage low.

CPU usage with this addition was down to <1% in testing.
B"H view1090: do not use pthreads

It *seems* that pthreads are used in dump1090 for I/! sample reading
only. So view1090 need not use pthread_exit.
Update dump1090.c
Create convienience.h
Create convienience.c
Update dump1090.c
Update dump1090.c
Delete convienience.h
Delete convienience.c
Update dump1090.c
Merge pull request #22 from matthewbrandes/master

Allows choosing device by serial number rather than device index which can change.
Merge pull request #21 from hhm0/view1090_net_mods

View1090 net mods
B"H decoding: use last air position to calculate ground position