[Gpsbabel-code] Added new saroute options

[Ron P. <ro...@pa...>]

I've just committed a couple of options I found more-or-less useful
for the saroute input filter.  They are:

turns_only: keeps only the routepoints corresponding to turns and
the start/stop point.  This nicely dovetails with the itinerary
from Street Atlas, which could be very useful in a cross-country
sort of drive ("It is 3.48 miles to your next turn, Dave.")

turns_important: not as strong as turns_only, this marks the turns
as "important" for the route simplification filter.  This ensures
that they are kept until there are no other points left.

As part of this update, I've added a route_priority field to the
waypoint structure.  This is initialized to 0, which is the lowest
priority.  The higher (numerically) the priority, the longer the
waypoint will survive the simplification process.  Conceptually,
the start and end waypoints of a route have infinite priority,
regardless of the value of their route_priority fields.