Note: it changes the interface slightly: instead uf -dualgps, use -e now.

This also fixes 2 bugs:
- nullptr access when doing 'gps status' with fake gps running
- close(fd) was called on an uninitialized fd when gps fake was running

fixes the compile error:
error: 'print_usage' defined but not used [-Werror=unused-function]

Also changes the order of the arguments for consistency.