Support "virtual shortcuts" to directory configured in SCFG->File Areas

As an alternative to directory aliases configured in ftpalias.cfg and/or
web_alias.ini, a sysop can specify a shortcut (virtual directory name) per
directory in SCFG->File Areas. These shortcuts will appears as top/root level
directories in the FTP and Web servers for quick user access to important
directories.

The advantages (over ftpalias.cfg / web_alias.ini) are:
- one place to configure
- FILE_FTP_PATH and FILE_WEB_PATH @-codes will use the shortcut
  (usually a shorter, preferred virtual path to the directory)

* New JS file_area.dir[].vshortcut property