sysvinit: downgrade ALTERNATIVE_PRIORITY[mountpoint]

sysvinit: downgrade ALTERNATIVE_PRIORITY[mountpoint]

sysvinit is objectively less maintained than util-linux or busybox, each
of which may supply its own mountpoint implementation. Adjust the
ALTERNATIVE_PRIORITY to select the sysvinit implementation as the last
resort.

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Ioan-Adrian Ratiu <adrian.ratiu@ni.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>