Fix unpackfs wrongly excluding separately-mounted partitions.

mount: Remember the extraMounts (and extraMountsEfi, if applicable) in
the global storage (as "extraMounts").

unpackfs: Read the extra mounts from the global storage to generate the
exclude list instead of trying to detect it from the "mount" command's
output, because the latter also includes normally-mounted destination
partitions.

This makes having separate partitions for non-/ mountpoints work again.