Another joint effort with Gerhard. Quit pretending that we have microsecond

Another joint effort with Gerhard.  Quit pretending that we have microsecond
support.  Be more honest and call it millisecond everywhere.  Remove more
explicit accesses to microseconds member.  Still not quite zero, but getting
closer and this is a good checkpoint.

git-svn-id: http://gpsbabel.googlecode.com/svn/trunk@4461 f51c46e8-681c-474f-0cfe-069cfd0219fb