[Gpsbabel-code] short names in gpx writer.

[tsteven4 <tst...@gm...>]

This change is motivated by the recent gpsbabel-misc thread "short 
names" started by R....@ex....

This is a change to the gpx write to support shortnames for gpx:rtept 
and gpx:trkpt.  It also changes the shortname handling for gpx:waypt to 
use the common routine mkshort_from_wpt.  I note that the fields 
searched and the search order used in mkshort_from_wpt is a bit 
different than what gpx.c has done in the past for gpx:wpt, but I think 
consistent handling is the correct thing to do.