add smime support to VM

[Arik]

This is a placeholder for adding S/MIME support to VM. This capability is now included in message-mode and gnus and many other mailers but not VM. I have implemented smime support in VM with the new mime encoding code (after trunk revision 1165), both for viewing (decryption and verification) and for sending with VM Mail mode (since I'm not sure if we are planning on going to message-mode any time soon). See branch smime for details (sorry about the commits, I had done some of this before 1165 and so pulled changes from there in pieces).

Support uses the smime package that comes with emacs and requires some setup to get going (which ~should~ not affect users not intending to view/compose smime encoded messages). The essentials are the variables smime-keys and smime-certificate-directory. Beyond that, to automatically view smime encrypted messages simply adding "application/pkcs7-mime" or "application/x-pkcs7-mime" will do the trick, otherwise clicking the mime button will do the trick.

Info docs still need to be added, which will explain down to the basics of generating keys , making CA and setting up the smime.el variables, but for the time being if anyone wants to have a go at the new addition let me know what you think. The banners after decryption/signing are sort of a temp measure until something better comes along, but an indication is necessary. The same goes for composing (try M-x vm-smime-sign-encrypt-message) where I just added something to the mode-line mode-name indicator. Anyhow, any questions also let me know.

Thanks,
-Arik