accept-operation: use SOCK_* flags, not O_*

That's how accept is defined and on some systems (OpenBSD), the
different flags do have different values.

As SOCK_CLOEXEC does exist on guile-2.2, we can use it by default
unconditionally.

fibers.texi

@@ -826,20 +826,17 @@ of these is implemented yet at time of writing.
 In the meantime, don't change the ‘less deep’ waiter while
 @code{with-read-waiter} / @code{with-write-waiter} is doing its thing!
 
-@defun accept-operation port [flags=(logior O_NONBLOCK O_CLOEXEC)]
+@defun accept-operation port [flags=(logior SOCK_NONBLOCK SOCK_CLOEXEC)]
 Like @code{(accept port flags)}, but as an operation.  Unlike
-@code{accept}, @code{O_NONBLOCK} is included in the default flags,
+@code{accept}, @code{SOCK_NONBLOCK} is included in the default flags,
 which makes the accepted port non-blocking and hence suitable for
 use with Fibers.
 
-The flag @code{O_CLOEXEC} is also included (if available).  Without
-this flag, ports would be leaked to subprocesses by default, which
-usually is at best inefficient and at worst a security problem.  But
-rarely, ports are intended to be leaked to subprocesses, in which
-case you could remove this flag.
-
-Fibers doesn't emulate @code{O_CLOEXEC} when unavailable, because it
-would not be entirely equivalent in case of parallelism.
+The flag @code{SOCK_CLOEXEC} is also included.  Without this flag,
+ports would be leaked to subprocesses by default, which usually is at
+best inefficient and at worst a security problem.  But rarely, ports
+are intended to be leaked to subprocesses, in which case you could
+remove this flag.
 @end defun
 
 @node REPL Commands

fibers/io-wakeup.scm

@@ -154,25 +154,17 @@ The write waiter at the current depth (relative to the invocation of
 the returned thunk) may not be changed while the thunk is being invoked."
   (with-x-waiting-is-failure port current-write-waiter try-fn))
 
-(define O_CLOEXEC*
-  (if (defined? 'O_CLOEXEC)
-      O_CLOEXEC ; doesn't exist on guile-2.2
-      0))
-
-(define* (accept-operation port #:key (flags (logior O_CLOEXEC* O_NONBLOCK)))
+(define* (accept-operation port #:key (flags (logior SOCK_CLOEXEC SOCK_NONBLOCK)))
   "Like '(accept port flags)', but as an operation.  Unlike
-'accept', 'O_NONBLOCK' is included in the default flags,
+'accept', 'SOCK_NONBLOCK' is included in the default flags,
 which makes the accepted port non-blocking and hence suitable for
 use with Fibers.
 
-The flag 'O_CLOEXEC' is also included (if available).  Without
-this flag, ports would be leaked to subprocesses by default, which
-usually is at best inefficient and at worst a security problem.  But
-rarely, ports are intended to be leaked to subprocesses, in which
-case you could remove this flag.
-
-Fibers doesn't emulate 'O_CLOEXEC' when unavailable, because it
-would not be entirely equivalent in case of parallelism."
+The flag 'SOCK_CLOEXEC' is also included.  Without this flag, ports
+would be leaked to subprocesses by default, which usually is at best
+inefficient and at worst a security problem.  But rarely, ports are
+intended to be leaked to subprocesses, in which case you could remove
+this flag."
   (define (try)
     (let ((new (accept port flags)))
       (and new (lambda () (values new)))))