more owornmask sanity_check

more owornmask sanity_check

Fix the problem with erroneously detecting wielded or quivered
chained ball which legitimately has more than one mask bit set.

Add an additional check for valid wornmask of an item somehow worn
in an invalid slot (such as an amulet in one of the ring slots).
Only lightly tested; it really needs debugger assistance to force
various invalid situations but I don't know gdb well enough for that.