security: E2EE and transport correctness (issue #147)

T04: promote security events to warnings — SASL auth failure, TLS
handshake failure, cert-failure details, see-other-host redirect;
DISABLE_TLS, TRUST_TLS and LEGACY_AUTH get a log warning plus a
console notice (REQ-LOG-01, REQ-LOG-02, REQ-AUTH-03)

T05: warn when a session ends up unencrypted without the user having
asked for it; refuse in-band registration on an unencrypted stream;
warn on each HTTP transfer with certificate verification disabled
(REQ-CRY-03, REQ-CFG-01)

T06: pin the update check to https with peer/host verification and no
redirects; strict N.N.N parser for the fetched version, which is
untrusted network input (REQ-VUL-02)

T09: no plaintext logging on failed MUC OMEMO sends; OTR opportunistic
first message passes allow_unencrypted_message(); get_random_string()
draws from a CSPRNG without modulo bias; guard the identity-key length
decrement against unsigned underflow (REQ-CRY-01, REQ-CRY-02,
REQ-CRY-07, REQ-MEM-05)

REQ-VUL-03 and REQ-CRY-09 are already satisfied on master and are
left unchanged.

The console warnings use cons_show_warning() from #87, so this needs
that change in master first.

src/otr/otr.c

@@ -22,6 +22,7 @@
 #include "config/files.h"
 #include "otr/otr.h"
 #include "otr/otrlib.h"
+#include "event/client_events.h"
 #include "ui/ui.h"
 #include "ui/window_list.h"
 #include "xmpp/chat_session.h"
@@ -331,6 +332,10 @@ otr_on_message_send(ProfChatWin* chatwin, const char* const message, gboolean re
 
     // tag and send for policy opportunistic
     if (policy == PROF_OTRPOLICY_OPPORTUNISTIC) {
+        // the tagged first message is still plaintext on the wire
+        if (!allow_unencrypted_message(chatwin, message)) {
+            return TRUE;
+        }
         auto_char char* otr_tagged_msg = otr_tag_message(message);
         id = message_send_chat_otr(chatwin->barejid, otr_tagged_msg, request_receipt, replace_id);
         chatwin_outgoing_msg(chatwin, message, id, PROF_MSG_ENC_NONE, request_receipt, replace_id);