temporary file handling tweaks related to Fh

restore this package as an empty package that CGI::File::Temp can
subclass so any existing code checking that the filehandle isa Fh
will still function correctly

split out the Fh and CGI::File::Temp packages into their own files
and add these to the MANIFEST, tweak perldoc slightly to avoid any
confusion regarding what package(s) still exist for temp files

bump version to next DEV release