xmlsec-nss: CERT_FindCertBySubjectKeyID does not work in all cases

[lsh123]

NSS bug: http://bugzilla.mozilla.org/show_bug.cgi?id=211051

Added code to search for certs in the imported list, still would love to have proper NSS support.

Migrated from: https://bugzilla.gnome.org/show_bug.cgi?id=118632