backup tool support kernelSu (beta)

#pumpkin_disclaimer

based on
8.3.1

* fixes cache being ignored despite the preference

* this is a test apk to test a new method to become mount master
* it should also work with kernelSU
* it should work with phhSU
* su doesn't need --mount-master
* please report if it works with other su/root implementations
* It uses nsenter -t 1 -m sh
which means, running a [sh]ell [t]aking the [m]ount namespace from the init process pid=1

[EDIT:
nsenter method is now tested:
* once for kernelSU
* once for phhSU
* at least two users with magisk
* please report if you have another su
]

this animal, I call him masbro😁

new lcd🥹

It feels like just yesterday the upstream was 308 and now it's 309💀