daemon/proxyv2: move PROXY protocol into its own layer

daemon/proxyv2: move PROXY protocol into its own layer

Previously, PROXYv2 handling was partially implemented in the `io.c`
unit in the `_TCP` and `_UDP` protocol layers, which technically made
very little sense. This commit moves this handling into separate
`_PROXYV2_DGRAM` and `_PROXYV2_STREAM` protocol layers, basically
encapsulating the handling of proxies in the `proxyv2.c` unit.

This commit also makes the PROXYv2 stream layer only support
`PROTOLAYER_PAYLOAD_WIRE_BUF` on its input, as other payload types were
unused and untested in this context.