Add support for hooks-cleanup

The pmOS initramfs will execute scripts in this dir, so let's add a way
to install them in the archive!

cmd/mkinitfs/main.go

@@ -111,6 +111,8 @@ func main() {
 		hookfiles.New("/etc/mkinitfs/files"),
 		hookscripts.New("/usr/share/mkinitfs/hooks", "/hooks"),
 		hookscripts.New("/etc/mkinitfs/hooks", "/hooks"),
+		hookscripts.New("/usr/share/mkinitfs/hooks-cleanup", "/hooks-cleanup"),
+		hookscripts.New("/etc/mkinitfs/hooks-cleanup", "/hooks-cleanup"),
 		modules.New("/usr/share/mkinitfs/modules"),
 		modules.New("/etc/mkinitfs/modules"),
 	})

doc/mkinitfs.1.scd

@@ -134,7 +134,8 @@ create/manage. mkinitfs reads configuration from */usr/share/mkinitfs* first, an
 	skipped.
 
 ## /usr/share/mkinitfs/hooks, /etc/mkinitfs/hooks
-## /usr/share/mkinitfs/hooks-extra*, /etc/mkinitfs/hooks-extra
+## /usr/share/mkinitfs/hooks-cleanup, /etc/mkinitfs/hooks-cleanup
+## /usr/share/mkinitfs/hooks-extra, /etc/mkinitfs/hooks-extra
 
 	Any files listed under these directories are copied as-is into the
 	relevant archives. Hooks are generally script files, but how they are