T5217: Add firewall synproxy

Add ability to SYNPROXY connections
It is useful to protect against TCP SYN flood attacks and port-scanners

set firewall global-options syn-cookies 'enable'
set firewall ipv4 input filter rule 10 action 'synproxy'
set firewall ipv4 input filter rule 10 destination port '22'
set firewall ipv4 input filter rule 10 inbound-interface interface-name 'eth1'
set firewall ipv4 input filter rule 10 protocol 'tcp'
set firewall ipv4 input filter rule 10 synproxy tcp mss '1460'
set firewall ipv4 input filter rule 10 synproxy tcp window-scale '7'

(cherry picked from commit bdad4e046872e054ec7783b2f04b73a8a690a045)

1. Conflicts:
2. interface-definitions/include/firewall/action-forward.xml.i
3. interface-definitions/include/firewall/action.xml.i
4. python/vyos/firewall.py
5. src/conf_mode/firewall.py