a better OS_error

[Sam S. <sd...@gn...>]

Marco Baringer <em...@be...> suggested that we make it possible to use
CLCS (the Common Lisp Condition System) to differentiate between
various OS errors, e.g.

(handler-case (chmod "foo" 0700)
  (OS:EPERM (co) ...))

This will require much unification between various err*.d files (i.e.,
this EPERM symbol must be the same on w32, unix, amiga &c).

A cheaper option would be be to add an OS-ERRNO slot to the condition
object and export functions CONDITION-ERRNO (returning NIL for non-OS
errors) and ERRNO-MESSAGE (see errunix.d:get_errormsg()).

What do people think?

-- 
Sam Steingold (http://www.podval.org/~sds) running RedHat7.2 GNU/Linux
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