tcp: do not assign TCP flags to pseudopackets

Previously pseudopackets were assigned with ACK flag which falsely turned
"SYN" flows to "SYN/ACK" flows when Suricata ran with raw content-matching
rules. The problem occured during the flow timeout or Suricata shutdown,
essentially, when the flow was being kicked out (with a pseudopacket).

When Suricata ran without raw content-matching rules (the ruleset did not
contain content matching keywords or it only contained keywords that are
app-layer content-matching) then raw stream reassembly tracking is turned off
(SignatureHasStreamContent()).
This in turn disabled a check in StreamNeedsReassembly() and the right edge
was not checked with the raw stream progress. In turn, it did not generate
a pseudopacket that would go through the detection engine. Suricata with
raw content-matching keywords would therefore on a flow with SYN packet only
return STREAM_HAS_UNPROCESSED_SEGMENTS_NEED_ONLY_DETECTION which would generate
the pseudopacket.

In Suricata versions <= 6.0.x, the flow output was correct because
only the commit 1bb6f44ff0 started to
differentiate the right edge calculation between the raw and application
layer streams. The older Suricata versions used only the application layer
right edge equation and therefore did not generate a pseudopacket.

Ticket: #6733

src/flow-timeout.c

@@ -211,7 +211,7 @@ static inline Packet *FlowForceReassemblyPseudoPacketSetup(
     }
 
     p->tcph->th_offx2 = 0x50;
-    p->tcph->th_flags |= TH_ACK;
+    p->tcph->th_flags = 0;
     p->tcph->th_win = 10;
     p->tcph->th_urp = 0;