magicsock: set the don't fragment sockopt (#8715)

This sets the Don't Fragment flag, for now behind the
TS_DEBUG_ENABLE_PMTUD envknob.

Updates #311.

Signed-off-by: Val <valerie@tailscale.com>
Signed-off-by: salman <salman@tailscale.com>

wgengine/magicsock/debugknobs_stubs.go

@@ -20,6 +20,7 @@ func debugAlwaysDERP() bool            { return false }
 func debugUseDERPHTTP() bool           { return false }
 func debugEnableSilentDisco() bool     { return false }
 func debugSendCallMeUnknownPeer() bool { return false }
+func debugPMTUD() bool                 { return false }
 func debugUseDERPAddr() string         { return "" }
 func debugUseDerpRouteEnv() string     { return "" }
 func debugUseDerpRoute() opt.Bool      { return "" }