modify the way update-binary mounts /system

[FAB1150]

I'm extremely new to magisk modules (in fact, this is the first time Iook into one), so excuse the ignorance.

This PR should solve issue #16 as it mounts /system as it does in https://github.com/nathanchance/magisk-modules/blob/master/META-INF/com/google/android/update-binary .

This removes the systemless check (and mounts /data twice), but it works if installed from the magisk app module manager. Probably needs some modification before merging

[FAB1150]

It indeed could be modified before merging. It now mounts /system in rw if $SYSTEMLESS is false like before, and doesn't mount a bunch of unneeded stuff.

This should also solve issue https://github.com/Magisk-Modules-Repo/IPControl/issues/11, looking into it.