fix #H5462 - failed F-attack gives misleading msg

Report was for 'F' followed by '.' reporting "cmdassist: Invalid
direction key!" and then by a direction grid (which happened to
include '.' for self).  That behavior applied for all the movement
prefix keys ('m', 'G', &c).  When 'cmdassist' was off, "F." would
yield "Unknown command 'F.'." instead.

Now you'll get "You can't fight yourself.", either instead of the
"invalid direction key" part of cmdassist feedback (followed by a
direction grid which excludes up, down, and self since they aren't
applicable for prefix keys) or of the "unknown command" result.
Likewise, "You can't run upward." or "You can't rush downward."
for "G<" and "g>", respectively.

doc/fixes36.1

@@ -380,6 +380,8 @@ levitation vs encumbrance message sequencing issues:  putting on boots of
 	of encumbrance until player took another action
 removing a blindfold with 'A' took two turns, with 'R' (and 'T') only one,
 	and could result in a panic if the blindfold was stolen during removal
+cmdassist help for movement prefix followed by invalid direction was strange
+	when the direction was up, down, or self disallowed for that prefix
 
 
 Fixes to Post-3.6.0 Problems that Were Exposed Via git Repository