wgengine/magicsock: replace CanPMTUD() with ShouldPMTUD()

Replace CanPMTUD() with ShouldPMTUD() to check if peer path MTU discovery should
be enabled, in preparation for adding support for enabling/disabling peer MTU
dynamically.

Updated #311

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

wgengine/magicsock/debugknobs_stubs.go

@@ -20,10 +20,11 @@ func debugAlwaysDERP() bool            { return false }
 func debugUseDERPHTTP() bool           { return false }
 func debugEnableSilentDisco() bool     { return false }
 func debugSendCallMeUnknownPeer() bool { return false }
-func debugEnablePMTUD() bool           { return false }
+func debugPMTUD() bool                 { return false }
 func debugUseDERPAddr() string         { return "" }
 func debugUseDerpRouteEnv() string     { return "" }
 func debugUseDerpRoute() opt.Bool      { return "" }
+func debugEnablePMTUD() opt.Bool       { return "" }
 func debugRingBufferMaxSizeBytes() int { return 0 }
 func inTest() bool                     { return false }
 func debugPeerMap() bool               { return false }