Turn our socketpair() replacement into its own function This patch splits the formerly windows-only case of evutil_socketpair() into an (internal-use-only) function named evutil_ersatz_socketpair(), and makes it build and work right on non-Windows hosts. We need this for convenience to test sendfile on solaris, where socketpair can't give you an AF_INET pair, and sendfile() won't work on AF_UNIX.