fix close_inherited_fds_slow

The intent of close_inherited_fds_slow is to mimick closefrom() in the
posix_spawn API, i.e. schedule the closing of all fds >= 3 after fork().

However, the way this is implemented in non-linux OSes is to schedule
the closing of 3..1023.  This fails due to EBADF in posix_spawn after
the fork() and makes it exit(127).

see: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=61095

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