gitian: add a gitian-win-signer descriptor

This is exactly like the current OSX signing process.

osslsigncode has been patched to detach and re-attach Windows signatures.
The changes can be seen here: https://github.com/theuni/osslsigncode/commits/attach-signature

There's a pull-request open upstream for the changes:
https://sourceforge.net/p/osslsigncode/osslsigncode/merge-requests/3/

This work has been back-ported to the stable 1.7.1 release of osslsigncode, so
that a smaller patch can be reviewed.