procd: create /dev/fd symlink

This is needed for ksh/bash style process substitution such as
<(command) and >(command) which was introduced in ash as of busybox
version 1.34.0 to work.

Signed-off-by: Erik Karlsson <erik.karlsson@genexis.eu>

package/system/procd/files/hotplug.json

@@ -11,6 +11,7 @@
 						[ "eq", "DEVNAME", "null" ],
 						[
 							[ "makedev", "/dev/%DEVNAME%", "0666" ],
+							[ "exec", "/bin/ln", "-s", "/proc/self/fd", "/dev/fd" ],
 							[ "exec", "/bin/ln", "-s", "/proc/self/fd/0", "/dev/stdin" ],
 							[ "exec", "/bin/ln", "-s", "/proc/self/fd/1", "/dev/stdout" ],
 							[ "exec", "/bin/ln", "-s", "/proc/self/fd/2", "/dev/stderr" ],