m-prefix groundwork (trunk only)

m-prefix groundwork (trunk only)

     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).