In this branch, I take a different approach from master, which currently tries to treat ports and file descriptors more or less equally.  Here file descriptors are treated as useful only at the edges of a Scheme program: (a) startup, when we may have been passed a fd from our predecessor program, (b) when we do a fork/exec/spawn and have to connect specific fds to our successor program, and (c) when we invoke something through the FFI.

As a result, section 3.2 is reduced to just seven procedures:  the four fdes->port procedures, port->fdes, dup->fdes, and close-fdes.  In addition, the other procedures in this branch no longer accept fds, only ports and in some cases filenames (which merge things like stat() and fstat()).  Everything else I/O-related either already exists in the (scheme file advanced) pre-SRFI at <https://bitbucket.org/cowan/r7rs-wg1-infra/src/default/FilesAdvancedCowan.md> or has been copied there as part of creating the branch.  That library basically provides everything R6RS does, plus make-pipe and the two select procedures from the master branch.

I'm not making this branch draft 3 yet, because it's pretty substantial and I think it deserves some discussion first.