#
# Disallow a unit (slave) from moving when it gets away from another unit's
# (master) movement range.
#
# * We set the slave's remaining MP to 0 on each turn refresh unless it is
#   within range of its master.
#
# * We set the slave's remaining MP to 0 on moveto whenever it leaves its
#   master's range. We don't care how far away from its master's range it can
#   move.
#
# * On each case above, we store the slave's previous remaining MP in a unit
#   variable so we can restore them once the master moves so that the slave
#   is within his movement range again.
#