implement in-place update

[programminghoch10]

• current INSTALLEDMODPATH seems to be compatible with KSU and APATCH
• use of exit 0 at the end is dirty, but required in order for magisk to not create the update file
• test -w /system/etc/hosts works properly on LineageOS bash, but the Magisk standalone ash does not detect if a file is filesystem-writable but the enclosing filesystem itself is not mounted writable