[VM] Key bindings - minor reorganization

[Uday Reddy]

I have finally gathered up enough courage to do something about VM key 
bindings.  VM uses up almost all keys that can be typed easily and 
assigns them to some function or other.  These keys get documented in 
the manual and become standardized, blocking future expansion and user 
customization.  So, the first step seems to be to remove some of the 
inessential key bindings from the standard VM set-up and let users bind 
them as they wish.  There will be a function `vm-optional-keys' which 
will reinstall all the key bindings that will be removed.  It can be 
easily invoked from the vm-preferences-file so that users can retain the 
old bindings if they wish.

My general principles behind the decisions are:

- It is desirable for a package to bind as few keys as possible so that 
the users will have the option of binding them or not, as they see fit.

- Commands and key bindings that are not documented in the manual will 
become optional.

- Commands that are used infrequently will be made optional.

- Multiple key bindings for the same commands will become optional.

With this underlying rationale, here is the list of key bindings I have 
come up with for moving to the optional category:

<    beginning-of-message   Not documented
>    end-of-message         Not documented
b    vm-scroll-backward     Duplicate key binding
=    vm-summarize           Redundant, `h' serves the purpose
i    vm-iconify-frame       Redundant, C-x C-z serves the purpose
!    shell-command          Redundant, M-! serves the purpose
e    vm-edit-message        Infrequent, and also mildly dangerous
L    vm-load-init-file      Infrequent
M-l  vm-edit-init-file      Infrequent and redundant
%    vm-change-folder-type  Infrequent
*    vm-burst-digest        Infrequent for most users
a    vm-set-message-attributes  Infrequent for most users
w    vm-save-message-sans-headers Infrequent for most users
M-g  vm-goto-message        Duplicate key binding

Please let me know if you seriously disagree with any of these decisions.

Cheers,
Uday