Fragmentation handling improvements

[Ralith]

Fixes #664.

Some questions following from this work:

• Should MTU discovery configuration be on the endpoint, rather than the connection? It's hard to imagine a case where a user might want to vary this per-connection, and usually the main concern is whether the common socket has DONTFRAG set. On the other hand, the reduction in complexity from moving it to the endpoint seems slight. I guess you could use a single endpoint to communicate both on a jumbo-frame-capable LAN and also the internet...
• Setting IP_DONTFRAG might fail pre-Big Sur (version 11). As written, this PR will make endpoint creation fail if this occurs, probably with EINVAL. Are older macOS builds worth supporting?
• Can we reduce the amount of work required from users to create a endpoint or connection with the recommended configuration for their platform?

[Ralith]

According to https://en.wikipedia.org/wiki/MacOS_version_history, macOS <11 is officially unsupported, at least.

[Ralith]

I've improved MTUD configuration ergonomics by adding an argument to quinn_proto::Connection::new which quinn uses to clobber the user-specified MTUD configuration on unsupported backends.

[djc]

Are older macOS builds worth supporting?

Let's default to no and see if people complain?

[djc]

Nice!

[complexspaces]

FWIW, rustc and the standard library still supports all the way back to macOS 10.7. I think the approach of waiting to see is fine, but I figured it was worth mentioning since there are a lot of users who are still on the 10.x line for reasons outside their control and this change would preclude the use of the crate in builds with a deployment target set to even 10.15 (out of support for less then a year).

[djc]

@complexspaces thanks for following up! Is Quinn in use (perhaps indirectly) at 1P?

since there are a lot of users who are still on the 10.x line for reasons outside their control

Can you quality "a lot of users" a bit more? Presumably this is the 1P population, can you disclose a rough percentage?

builds with a deployment target set to even 10.15 (out of support for less then a year).

In my mind that's still likely to be a pretty small population... For context, 10.15 (Catalina) is the version released in Oct 2019, so about 2.5 years ago at this point.