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/http/http.go

@@ -17,7 +17,7 @@ func GetClientIP(req *http.Request) net.IP {
 	if sip == "" {
 		ss := strings.Split(req.Header.Get("X-Forwarded-For"), ",")
 		if len(ss) > 0 && ss[0] != "" {
-			sip = ss[0]
+			sip = strings.TrimSpace(ss[0])
 		}
 	}
 	if sip == "" {
@@ -53,7 +53,7 @@ func (p *Body) Read(b []byte) (n int, err error) {
 			b = b[:p.recordSize]
 		}
 		p.buf.Write(b)
-		p.recordSize -= n
+		p.recordSize -= len(b)
 	}
 
 	return