SocketBindAllow extend to more IP protocols

[coelner]

Component

other

Is your feature request related to a problem? Please describe

I tried to limit a service to sctp.

Describe the solution you'd like

Extend this transport-protocol := { tcp | udp } to a more flexibel approach. tcp|udp seems useful for most services. Instead of adding all abbreviations, allow using the IP protocol number. Maybe in hex, to recognize that this is not the port number.

ref: https://en.wikipedia.org/wiki/List_of_IP_protocol_numbers

Describe alternatives you've considered

Just using a firewall rule.

The systemd version you checked that didn't have the feature you are asking for

249