* enable DPLPMTUD on macOS dual-stack sockets
https://datatracker.ietf.org/doc/draft-seemann-tsvwg-udp-fragmentation/
contains details on how IP fragmentation is handled on different
platforms.
* only enable DF on macOS Sequoia (and newer) dual-stack sockets
* fix macOS version numbers
* fix comment in MTU integration test
* skip dual-stack test on old macOS versions
