v3 of fixing signature verification

[Daniel Kahn Gillmor]

Bremner pointed out to me that v2 of this patch doesn't work with 2.6,
and that the name of the function should be
g_mime_certificate_get_valid_userid, instead of
g_mime_get_certificate_valid_userid.

Also, he asked that it be built against the "release" branch, since
it's important there, too.

So v3 is built against the release branch, though it is also trivial
to cherry-pick or merge into master.