new feature: slirp_remove_guestfwd
This merge proposal adds a useful feature for the VDE projects (and not only).
remove (guest addr) forward services: While slirp_remove_hostfwd removes a forward service activated by slirp_add_hostfwd, slirp_add_exec and slirp_add_guestfwd do not have a remove function. The proposed patch adds the function "slirp_remove_guestfwd".