Improve platform support by checking /dev/fd instead of /proc/self/fd

The virtual /proc/self/fd is Linux-only, while the virtual /dev/fd is
available on more platforms. On Linux, the latter is simply a symlink to
the former, so this shouldn't affect existing installations.

Author: clue

src/Io/functions.php

@@ -11,9 +11,9 @@
  * @return array
  * @internal
  */
-function fds($path = '/proc/self/fd')
+function fds($path = '/dev/fd')
 {
-    // try to get list of all open FDs (Linux only) or simply assume range 0-1024 (FD_SETSIZE)
+    // try to get list of all open FDs (Linux/Mac and others) or simply assume range 0-1024 (FD_SETSIZE)
     $fds = @\scandir($path);
 
     return $fds !== false ? $fds : \range(0, 1024);