fix(dialer): correct inverted setMark on FreeBSD/OpenBSD

Fix setMark early-return conditions that were inverted relative to
Linux: on FreeBSD (SO_USER_COOKIE) and OpenBSD (SO_RTABLE), a
non-zero mark was short-circuited instead of applied, disabling
socket marking entirely.

Also fix GetClientIP to trim leading whitespace from X-Forwarded-For
entries (per RFC 7239), and fix Body.Read to subtract len(b) instead
of n from recordSize when a single read exceeds the remaining quota.

Add unit tests for internal/net/ (addr, dialer, http, ip, net, pipe,
transport, resolve, proxyproto, udp).

internal/net/dialer/dialer_freebsd.go

@@ -9,7 +9,7 @@ func bindDevice(network, address string, fd uintptr, ifceName string) error {
 }
 
 func setMark(fd uintptr, mark int) error {
-	if mark != 0 {
+	if mark == 0 {
 		return nil
 	}
 	return unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_USER_COOKIE, mark)