net/interfaces: reduce & cleanup logs on iOS

We don't need a log line every time defaultRoute is read in the good
case, and we now only log default interface updates that are actually
changes.

Updates #3363

Signed-off-by: James Tucker <james@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f