support IP_MTU_DISCOVER

[milantracy]

Description

How to reproduce

# tracepath google.com
tracepath: IP_MTU_DISCOVER: Operation not supported

Is this feature related to a specific bug?

No response

Do you have a specific solution in mind?

No response

[pkit]

@milantracy do you need IP_MTU_DISCOVER flag to not fail, or you do actually need tracepath to find the path? The latter is kinda a security issue.
Fixing the flag doesn't look too hard as PMTU is supported. It just needs to correctly set DF and ignore mtu updates.