file-readable? and friends should raise &i/o-file-does-not-exist

[Andreas Rottmann]

The current behavior:

Ikarus Scheme version 0.0.4-rc1+, 64-bit (revision 1829, build 2009-07-28)
Copyright (c) 2006-2009 Abdulaziz Ghuloum

> (file-readable? "/not/there")
Exception trapped by debugger.
 Condition components:
   1. &error
   2. &who: file-readable?
   3. &message: "ENOENT: No such file or directory"
   4. &i/o-filename: "/not/there"

I think it would be better to raise `&i/o-file-does-not-exist-error',
instead of its supertype `&i/o-filename', as it would convey more
(machine-understandable) information to the caller.

[Andreas Rottmann (rotty)]

The attached patch should fix this for all procedures from ikarus.posix.ss.