Set up passes to run in a separate thread

This is a slight re-architecting of the system, although it
required fairly minimal actual code changes.  Every pass will now
have a thread dedicated to it.  This allows the main loop to coninue
following all satellites, and performing tasks such as TLE updates,
prioritization, and so on.
Set up passes to run in a separate thread

This is a slight re-architecting of the system, although it
required fairly minimal actual code changes.  Every pass will now
have a thread dedicated to it.  This allows the main loop to coninue
following all satellites, and performing tasks such as TLE updates,
prioritization, and so on.

This is needed for issues #3, #7 and #8
Merge branch 'threading' of github.com:kstripp/multitrack into threading
Changed tleupdate call to be automatic

Moved in to a function in libTLE which uses a static
variable to maintain knowledge of update interval.

Closes #3.
Added priority checks when new pass thread starts

There is now a priority field in the sat config file and
a routine that checks it every time a pass starts.  If a new
pass thread starts with a higher priority than the active one,
the active one is told to stop and it switches to the new one.
Otherwise the new one waits until the active pass is complete.

Refs #8.
Moved semaphore release to end of track_pass

This will hopefully help a bit with pass hand off.
Added priority to sample sat files