Get correct uri of a message for security status

We always use uri argument if it is available.
The uri argument is not supported by recent Enigmail.
https://sourceforge.net/p/enigmail/bugs/139/
Verify a message with attached signature

This feature doesn't work with recent version of Enigmail.
I fixed it to work with latest Enigmail. It may not work with older
version of Enigmail.
Verify multipart/signed message

Restore previous code for verifying multipart/signed message.
Merge pull request #732 from shirosaki/enigmail

Fix Enigmail features broken with recent Enigmail change