Re: largefile64 on ports

[Kevin Ryde]

Perceiving a distinct lack of enthusiasm for extending the port
descriptor I added the 64 bits just for fports, with scm_seek and
scm_truncate calling direct to fport functions on finding that type.
Exposing the same to applications can wait for another time, perhaps
when instantiating port descriptors is actually documented.

I added this to the ports section intro:

    All file access uses the "LFS" large file support functions
    when available, so files bigger than 2 Gbytes (2^31 bytes) can
    be read and written on a 32-bit system.

I would have put a cross reference to the glibc manual on the subject,
but there doesn't seem to be a summary of what it all means there
(only individual functions).

I was tempted to mention O_LARGEFILE and how you can turn it off with
fcntl, but I'm not sure if that's too much detail.  It should only be
needed if you open an fd in guile then pass it to an external library
which is 32-bit and is so sloppy that it doesn't check error returns
from seek().