Use case:
1) Config uses a negative home_sequence for synchronized
homing final move (used on some gantry configs).
Jogging of joints in the sequence is disabled
(recent modification).
2) User may want a method to jog joints prior to homing
without requiring a config stop/edit/restart.
With this commmit, joints configured for synchronised homing
by negative [JOINT_N]HOME_SEQUENCE values can be temporarily
allowed to joint jog by setting hal pins (ini.N.home_sequence)
to non-negative values. Non identity kinematics should be
used (trivkins kinstype=B for KINEMATICS_BOTH).
Note: as with all ini hal pins, changes are acknowledged
when not performing coordinated motion.
