largefile64

[Kevin Ryde]

I started a few minimal bits of 64-bit file support: in stat, lstat,
readdir, truncate-file and copy-file.

I don't have 4G of disk to test the copy-file, but it starts running
ok.  (It's not blindingly fast, maybe a bigger buffer would help.
It'd be nice if it was interruptable too, maybe SCM_TICK and some
dynwind protection on the fds, or something ...)

The next step will be the various operations on file descriptors.  I
think making them all 64-bits is a good thing.  The only visible
change will be that fds opened by guile are in largefile mode, so the
errors are slightly different if some application C code is not 64-bit
(return EOVERFLOW instead of the program killed by SIGXFSZ, or
something like that).

fports will come next.  I'm still thinking of adding seek64 and
truncate64 funcs to scm_ptob_descriptor, probably visible only when
_LARGEFILE64_SOURCE is defined, so an application can set those
if/when it wants.  Something internal-only and specific to fports
could be done though.