lcDiskfile::Open() uses fopen() to open a file. On Linux, this can open
a directory just fine (at least for reading). This is slightly problematic
when it is attempted to open the parts library $(sharedir)/library.bin
and the library is an unzipped directory hierarchy. It is first attempted
to open the path as a ZIP file. While opening the directory as a file is
successful, subsequent navigation in the open "file" fails.
Pretend that a directory is an empty file so that the ZIP file reader is
not tempted to navigate around in the "file" and so reports failure in a
deterministic manner.
We could have inserted the check for regular files in lcDiskFile::Open(),
but this burdens every file open request, which can happen thousands of
times when the parts library is extracted instead of in a ZIP file.
