New port interface

[Ian Grant <I.A.N.Grant@damtp.cam.ac.uk>]

There is an assumption in this that all ports are fports. For example
scm_putc will call scm_syserror (via the macro SCM_MAYBE_DRAIN_INPUT) if
an error happens when the port's seek function is called. This is not
always appropriate because the error may not be a system error. If it
isn't then we'll be issuing errors like "Error: Success".

We need a mechanism in the scm_ptobs interface to allow the underlying
port implementation to return error messages. Perhaps something as simple
as another function pointer, which in the case of fports could just call
strerror(errno).

Ian