Log pre-handshake packet drops using D_MULTI_DROPPED

Log pre-handshake packet drops using D_MULTI_DROPPED

We have a debug level packets dropped by the TLS layer - use that for this
packet drop too.  This changes this message from 'verb 3' to 'verb 4'
(which should result in less user reports about this almost always
harmless warning).

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180211101929.4535-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16477.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c215c58f2393e881e16f9805549316a1e257a682)