Renamed parser_fileset_define_common to parser_fileset_init_common
Files created by filebench start with FB_PREFIX

This helps up distinguish imported from generated files, within the same
fileset. Currently it is defined as "fb", but we should implement it as
an attribute of fileset/import as it may not suit specific users.
Simplified fileset population code

- Renamed fileset_populate_subdir to fileset_populate_define
- Renamed fileset_populate_leafdir to fileset_populate_dir
- Offloaded overlapping functionality of fileset_populate_subdir
  to fileset_populate_dir
Code support for import file/fileset functionality