Fix pgp and smime decryption in mutt_prepare_template().

Fix pgp and smime decryption in mutt_prepare_template().

Change the "combined" multipart decryption block to only work for pgp,
since mutt_is_multipart_encrypted() currently only checks for pgp
headers and it therefore only worked for pgp in the first place.

Fix the newhdr->security to be based on what that function returns,
instead of the "context" hdr passed in.

Add a smime decryption block below when iterating through the content.

Fix the application/pgp decryption block to assign to hdr->security
using the type of the app/pgp part instead of hdr->content.