Add support for 3 new loadable modules for file scanning/listing/viewing

Configured in SCFG->System->Loadable Modules:
Scan Dirs:      User scans one or more directories for (e.g. new) files
List Files:     User lists files within a file directory
View File Info: User views detailed information on files in a directory

This addresses/fixes feature request #521 for Nightfox

Will need to document the mode argument bit values on the wiki, but
it's the usual suspects: FL_* for scandirs and listfiles and FI_* for
fileinfo. The scandirs_mod will be passed an extra bool (0/1) arg that
indicates whether or not the user is scanning *all* directories.

Test results would be welcome