scripts: report on author emails that are mangled by the mailing list

scripts: report on author emails that are mangled by the mailing list

In some cases the Author: email address in patches submitted to the
list gets mangled such that it says

    John Doe via Qemu-devel <qemu-devel@nongnu.org>

This change is a result of workarounds for DMARC policies.

Subsystem maintainers accepting patches need to catch these and fix
them before sending pull requests, so a checkpatch.pl test is highly
desirable.

Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>