Use gflags to parse command line arguments for convert_imageset

In the dev branch, many new options have been added to the utility convert_imageset, which makes the old style parsing tedious and the usage hard to remember. This PR aims to replace positional argument parsing with `gflags`, now that it becomes a dependency.
ROOTFOLDER/ LISTFILE DB_NAME seems to me mandatory arguments, and thus not quite fit to be parsed by gflags altogether.