Add PMTUD feature

[Svedrin]

I'm wondering if we should add a PMTUD feature. That is, regularly figure out what the path MTU to a certain target is, and show that in the target list.

Work is underway to support this in icmplib:

• Add the ability to set the DF flag and get MTU size feedback from the ICMP protocol. ValentinBELYN/icmplib#71
• Feature/pmtu discovery ValentinBELYN/icmplib#64

Let's see if we can add a rogue implementation to MeshPing and then migrate once it's ready upstream.

[Svedrin]

Upstream issues have not seen activity in a while, so I'm not sure it makes sense to keep this open. We'll close for now and reopen.