core/mount: pass "-c" flag to /bin/umount (#6093)

"-c", which is short for "--no-canonicalize", tells /bin/umount
that the path name is canonical (no .. or symlinks etc).

systemd always uses a canonical name, so this flag is appropriate
for systemd to use.
Knowing that the path is canonical allows umount to avoid
some calls to lstat() on the path.

From v2.30 "-c" goes further and causes umount to avoid all
attempts to 'lstat()' (or similar) the path.  This is important
when automatically unmounting a filesystem, as lstat() can
hang indefinitely in some cases such as when an NFS server
is not accessible.

"-c" has been supported since util-linux 2.17 which is before the
earliest version supported by systemd.
So "-c" is safe to use now, and once util-linux v2.30 is in use,
it will allow mounts from non-responsive NFS servers to be
unmounted.