From a bug report: when riding,
#jump command and jumping spell didn't check whether steed is able to move.
This makes #jump command, either magic boots or knight's innate ability,
fail if the steed can't move. Spell still succeeds; its magic overrides
steed's immobility. Fix is trunk only since it relies on a function that
isn't in the branch.
Also, tweak hero movement to avoid an extra function call per move
when not riding.
2343e77eb0