A completely mechanical search and replace to swap the first argument

A completely mechanical search and replace to swap the first argument
for an xml_generic callback with a typdef that can be swapped at compile
time.

Yes, this is a hint of things to come to reduce future bulky merges.

git-svn-id: http://gpsbabel.googlecode.com/svn/trunk@4627 f51c46e8-681c-474f-0cfe-069cfd0219fb