Set up to be able to use the m command prefix to request menu mode
for commands other than just pickup. As a side-effect, a hardcoded ','
referencing that command is eliminated. Also, the command handling for
pickup was accepting the other prefix characters (F,g,G,M,numpad 5) as
synonyms for m; this reverts all but 'm' to movement modifiers only (it
may be reasonable to retain M and 5, but this doesn't).
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