bug#61095: possible misuse of posix_spawn API on non-linux OSes

Actually I can avoid the EBADF by checking that the fd is 'live' with
something like fstat:

[[[
Index: libguile/posix.c
--- libguile/posix.c.orig
+++ libguile/posix.c
@@ -1325,8 +1325,12 @@ SCM_DEFINE (scm_fork, "primitive-fork", 0, 0, 0,
 static void
 close_inherited_fds_slow (posix_spawn_file_actions_t *actions, int max_fd)
 {
-  while (--max_fd > 2)
-    posix_spawn_file_actions_addclose (actions, max_fd);
+  struct stat sb;
+  max_fd = getdtablecount();
+  while (--max_fd > 2) {
+    if (fstat(max_fd, &sb) != -1)
+      posix_spawn_file_actions_addclose (actions, max_fd);
+  }
 }
 
 static void

]]]

The regress passes and while this workaround may be temporarly
acceptable I -personally- don't like it much.  There's a reason guile
can't set CLOEXEC for all the file descriptors > 2 obtained via open,
socket, pipe, ... like perl -for example- does?