new feature: slirp_add_unix

This merge proposal adds a useful feature for the VDE projects (and not only).

forward to UNIX socket. (slirp_add_unix) It is similar to slirp_add_exec but instead of spawning a command it opens a UNIX stream socket. It is useful for example to allow X clients running on a VM or in a namespace to use the X-window server of the host. e.g. the diversion of 10.0.2.4:6000 to /tmp/.X11-unix/X0 allows all clients using -display 10.0.2.4:0 to run their graphical interface on the X-server :0 on the host

This MR includes the previous slirp_remove_guestfwd as it needs to be modified to be compatible with slirp_add_unix.