activate: get rid of redundant definiton of fd_cloexec()
activate: reduce log settings to normal
active: rework make_socket_fd() to be based on socket_address_listen()

Among other things this makes sure we set SO_REUSEADDR which is
immensely useful.
socket-proxyd: rework to support multiple sockets and splice()-based zero-copy network IO

This also drops --ignore-env, which can't really work anymore if we
allow multiple fds. Also adds support for pretty printing of peer
identities for debug purposes, and abstract namespace UNIX sockets. Also
ensures that we never take more connections than a certain limit.
mkdir: modernize header ifdefs