[CM] S7 integrated in TIC-80, and bug reports

[bil at ccrma.Stanford.EDU]

Do you need to disallow reading a file?  If it's just
creating or altering a file that needs to be blocked,
you could redirect fopen and fwrite (in s7.c) to
functions that raise an error.  I don't think s7 uses
creat, open (except with O_RDONLY), or write.  Also
build it with WITH_C_LOADER=0 (to disallow dynamic
loading of C object code), and maybe WITH_SYSTEM_EXTRAS=0.
Hmmm... as I type this, this seems interesting --
maybe I'll tackle it later today.  It might be
equally easy to disallow reading a file -- fread etc.
Oh, and for fopen, check the mode doesn't have "w" or "x"
or whatever else might change a file.  I'm probably
forgetting something obvious.

(There's also the sandbox procedure in stuff.scm, but
it's been years since I looked at it).