snet: Add MTU exceeded errors

[scrye]

snet currently does no MTU validation. This means that applications might be sending datagrams that are over the allowed length without ever being informed of this.

At the very least, snet should return an error when the MTU is exceeded.

[kormat]

A few comments:

• currently we do not enforce the MTU, as we are still reliant on over-sized UDP packets. That said, the BRs only allow 9KiB packets, everything longer gets truncated at the moment.
• this (naturally) only applies to datagram-based protocols
• an additional desired feature is to be able to query snet to determine the maximum payload size for a datagram.

[matzf]

currently we do not enforce the MTU, as we are still reliant on over-sized UDP packets. That said, the BRs only allow 9KiB packets, everything longer gets truncated at the moment.

We still allow fragmentation, so oversize datagrams do make it through. This used to be required for the control-plane rpcs. This is no longer the case with QUIC as the transport layer for the RPCs; so this can now be fixed without breaking the control plane.