Move RST handling into intercept_tcp
Move RST handling into intercept_tcp

Previously we had
- Endpoint.input_tcp: which handled RST and then called
  intercept_tcp_syn with a default forwarding option (connect to (ip,
  port))
- Endpoint.intercept_tcp_syn: also used by services like the HTTP proxy

The RST handling wasn't clear for the HTTP proxy etc, so rename
intercept_tcp_syn to intercept_tcp and ensure it handles RST packets
too.

Signed-off-by: David Scott <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@recoil.org>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit