shared: extract `socket_forward_new()` helper from socket-proxyd

shared: extract `socket_forward_new()` helper from socket-proxyd

This commit extracts the socket forwarding code from the existing
socket-proxyd into a new shared helper that will be used by the
varlinkctl protocol upgrade support code and is used as is in
the socket-proxyd.c.

It tries to keep the changes as small as possible, its mostly
renaming like:
* connection_create_pipes -> socket_forward_create_pipes
* connection_shovel -> socket_forward_shovel
* connection_enable_event_sources -> socket_forward_enable_event_sources
* traffic_cb -> socket_forward_traffic_cb

and a new socket_forward_new() that creates/starts the forwarding.

All log_error_errno() got downgraded to log_debug_errno().