This is used by the GS/OS Finder to conceal FINDER.DATA files.
Documented in "Exploring GS/OS and ProDOS 8" by Gary B. Little.
BASIC.SYSTEM's CAT/CATALOG are unaware of this, so the files are still
listed.
Note that the command is named "UNHIDE" rather than "SHOW" as that
sounds like a command to display file contents or something.
