P.S. It would be neat to do something like:

(codeset-message (errno ENOENT))

Since the procedure's arglist changes, this would take another SRFI
(which could be an alternative to this one, such that both SRFIs can
query the same underlying codesets).

But I can't find anything in R6RS to get the "name" of an enumeration.
Perhaps something like this would work:

(define-enumeration <errno>
   (EPERM
    ENOENT
    ESRCH
    EINTR
    EIO)
   errno)

(codeset-associate-enum 'errno (errno))

(codeset-message (errno ENOENT))