(mspools-folder-directory): Takes default value of ~/MAIL/ if

(mspools-folder-directory): Takes default value of ~/MAIL/ if
vm-folder-directory not bound.
(mspools-vm-system-mail): Customize the variable so that if the
environment variable $MAIL is not set, the user can easily set it.
(mspools-vm-system-mail-crash): Variable deleted; we now use
vm-crash-box.
(mspools-set-vm-spool-files): Error if mspools-folder-directory
hasn't been set.
(mspools-set-vm-spool-files): Use mspools-folder-directory rather
than vm-folder-directory.
(mspools-get-spool-files): Error if mspools-folder-directory is nil.
(mspools-get-spool-files): Add $ to regexp to ensure mspools-suffix
matches the end of the filename.
(mspools-mode-map): Bind n and p to next-line and previous-line.