Tailscale merges multiple ACL rules into fewer FilterRule entries
when they have identical SrcIPs and IPProto, combining their DstPorts
arrays. This change implements the same behavior in Headscale.
Add mergeFilterRules() which uses O(n) hash map lookup to merge rules
with identical keys. DstPorts are NOT deduplicated to match Tailscale
behavior.
Also fix DestsIsTheInternet() to handle merged filter rules where
TheInternet is combined with other destinations - now uses superset
check instead of equality check.
Updates #3036
0b1727c337