This commit introduces additional platform-dependent checking when the
kernel returns an error. Previously, the session is terminated when
PingFrame sends a discovery packet larger than the limit. With this
commit, the error is checked, and if it is "datagram too large", the
error is ignored.

Additionally,
1. This commit re-enables MTU discovery on Windows unless it
is disabled explicitly by user (Undo #3276),
2. Set IP_DONTFRAGMENT and IPV6_DONTFRAG with error checking on Windows,
   and
3. Set IP_MTU_DISCOVERY to PMTUDISC_DO for both IPv4 and IPv6 on Linux
   so that the kernel will return "message too long".

Fixes #3273 #3327

If the env QUIC_GO_DISABLE_RECEIVE_BUFFER_WARNING is set to true
(as defined by strconv.ParseBool()), the receive buffer warning
will not be printed out.

…CapablePacketConn (#3334)

* move set DF implementation to separate files & avoid the need for OOBCapablePacketConn

* merge err_size_* into conn_df_* & fix format

update for Go 1.18

* feat(http3): implement serving from quic.Listener

ServeListener method added to http3.Server allowing serving from an existing listener
ConfigureTLSConfig function added to http3 which should be used to create listeners meant for serving http3.

* docs(http3): add note about using ConfigureTLSConfig to ServeListener

* fix(http3): stop serving non-created listeners after Server.Close

* refactor(http3): return ErrServerClosed once server closes instead of context.Canceled

* feat(http3): close listeners from ServeListener as well

* fix(http3): fix logger not being setup during ServeListener

* test(http3): add unit tests for serving listeners

* test(http3): add tests for ConfigureTLSConfig

* test(http3): added server hotswapping integration test

* fix: race condition in listener tests

…eaders (#3352)

* feat: cache alt-svc headers and announce all listeners instead of just one

* feat: use Server.Addr for SetQuicHeaders if no port is available from listeners