Fix pivot handler to not consume all packets

[busterb]

Packet handlers should only return true if they consume a packet. Otherwise, they should return false so something else can consume it. This fixes port forwards by allowing the socket handler to see packets
that were otherwise being discarded in the pivot handler.

Fixes #9479

Verification

List the steps needed to make sure this thing works

• Start msfconsole
• Start a listener and get any Meterpreter session type. I primarily tested with:

./msfconsole -qx 'use multi/handler; set payload linux/x86/meterpreter_reverse_tcp; set lhost 192.168.56.1; run'

• At the Meterpreter prompt add a reverse listener: portfwd add -R -L 127.0.0.1 -l 8080 -p 8081
• In another terminal, start netcat listening on port 8080: nc -lk 8080
• In another terminal, connect to port 8081 and send/receive some bytes on port 8081: nc 127.0.0.1 8081
• Verify that bytes are sent and received.

Alternately, do anything that uses pivot functionality.

[busterb]

@bwatters-r7 another interesting case to see about adding to the payload regression tests

[busterb]

Thanks!

[allrosenthal-r7]

@busterb can you please write release notes for this PR?

[allrosenthal-r7]

Release Notes

The pivot handler was returning 'true' for any packet, even if it did not actually consume it, which led to the socket channel handler not being sent any packets at all. Now, if the pivot handler does not consume the packet, it returns 'false', which allows the socket handler to see packets that were otherwise being discarded in the pivot handler.