wgengine/magicsock: disable raw disco by default; add envknob to enable

Updates #13140

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ica85b2ac8ac7eab4ec5413b212f004aecc453279

wgengine/magicsock/magicsock.go

@@ -508,13 +508,13 @@ func NewConn(opts Options) (*Conn, error) {
 	if d4, err := c.listenRawDisco("ip4"); err == nil {
 		c.logf("[v1] using BPF disco receiver for IPv4")
 		c.closeDisco4 = d4
-	} else {
+	} else if !errors.Is(err, errors.ErrUnsupported) {
 		c.logf("[v1] couldn't create raw v4 disco listener, using regular listener instead: %v", err)
 	}
 	if d6, err := c.listenRawDisco("ip6"); err == nil {
 		c.logf("[v1] using BPF disco receiver for IPv6")
 		c.closeDisco6 = d6
-	} else {
+	} else if !errors.Is(err, errors.ErrUnsupported) {
 		c.logf("[v1] couldn't create raw v6 disco listener, using regular listener instead: %v", err)
 	}