When ACL rules don't specify a protocol, Headscale now defaults to
[TCP, UDP, ICMP, ICMPv6] instead of just [TCP, UDP], matching
Tailscale's behavior.
Also export protocol number constants (ProtocolTCP, ProtocolUDP, etc.)
for use in external test packages, renaming the string protocol
constants to ProtoNameTCP, ProtoNameUDP, etc. to avoid conflicts.
This resolves 78 ICMP-related TODOs in the Tailscale compatibility
tests, reducing the total from 165 to 87.
Updates #3036
f735502eae