[gforth] "string.fs" in included-files

[Josh Grams]

When gforth starts up, there is a bare "string.fs" in included-files.
Looking at the source, it seems to be hard-coded in.  This means that if
I have my own string.fs and try to `require` it, nothing happens, which
is bad.

On a similar note, on the Windows build, *all* of the filenames in
included-files are bare filenames with no absolute path, so this could
happen with quite a lot of common names...

--Josh