The switchNetns function was only defined for linux (dialer_netns.go,
build tag linux && !android) and android (dialer_android.go, stub).
This left darwin, freebsd, openbsd, windows, and other platforms with
an undefined symbol at dialer.go:55.
Add no-op stubs to all remaining platform files so the dialer package
compiles on every GOOS.
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).
ginuerzh