All files: replace explict calls to fopen with xfopen wrapper that

All files: replace explict calls to fopen with xfopen wrapper that
supports - as alias for std{in,out} and tests for error.

git-svn-id: http://gpsbabel.googlecode.com/svn/trunk@638 f51c46e8-681c-474f-0cfe-069cfd0219fb