Added support for a "list messages module" which is an optional module (Baja or

JS) which will be executed when a users uses the the (L)ist msgs command from
the "Reading" sub or "Reading E-mail" prompt.
The arguments passed to the module are: <sub-code> [which]  [usernum]  [lm_mode]
The additional optional arguments are the same values as would be passed to a
readmail_mod, if one were to be used instead.