Improve attachment menu for s/mime parts. (closes #113)

Improve attachment menu for s/mime parts. (closes #113)

Don't prompt for passphrase or getkeys, or set the ENCRYPT flag, for
OPAQUE types.

Don't recurse on "text" output from decrypt_mime().  There is no
reason to recurse on a text type.  Additionally, the
mutt_read_mime_header() will return an empty text type even if the
decode doesn't generate mime output.  In those cases, we want to show
the original attachment.