wgengine/magicsock: rename debugPMTUD() to debugEnablePMTUD()

Make the debugknob variable name for enabling peer path MTU discovery match the
env variable name.

Updates #311

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

wgengine/magicsock/peermtu_linux.go

@@ -30,5 +30,5 @@ func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
 }
 
 func CanPMTUD() bool {
-	return debugPMTUD() // only if the envknob is enabled, for now.
+	return debugEnablePMTUD() // only if the envknob is enabled, for now.
 }