Add /usr to non-unmountable list
Debians systemd has split-usr enabled as both Debian and in principle Apertis
support both a split and a merged-usr setup. This mostly adds search paths to
system (e.g. to look at both /bin/ and /usr/bin) however it also means that
if /usr is a mountpoint system will try to unmount it. Unfortunately this causes
issues at shutdown for systems with a merged-usr layout as basic libraries (e.g. libc)
are located in /usr making it impossible to unmount.
For Apertis we don't support /usr being a seperate partition, the only time
/usr is on a mountpoint is when using ostree images (where it is a bind mount
and which use a merged-usr layout). So also add /usr to the list of paths that
are considered unmountable even with split-usr support enabled in systemd.