1. Introduce new applet: imgtool for better separation from the main program
2. Actually mount the image and check statvfs for free space in the image
This shall eliminate any possible module installation failure from image resizing issues.
It's not important to check the return value of unlink(2) or even verify
that the file exists. If this code is running, it means the system has
rebooted, and thus the update file, if any, should be removed so that
MagiskManager doesn't keep displaying the same message. We also handle
this before we handle "disable" so that disabled modules don't keep
requesting a reboot to update.
