ngtcp2: Enable Linux GSO

[tatsuhiro-t]

Enable Linux GSO in ngtcp2 QUIC. In order to recover from the
EAGAIN/EWOULDBLOCK by sendmsg with multiple packets in one GSO write,
packet buffer is now held by struct quicsocket. GSO write might fail
in runtime depending on NIC. Disable GSO if sendmsg returns EIO.

[tatsuhiro-t]

-T 4GiB file on loopback interface against ngtcp2 server:

without GSO:

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 4096M  100  1080  100 4096M     84   321M  0:00:12  0:00:12 --:--:--  323M

with GSO:

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 4096M  100  1080  100 4096M    137   522M  0:00:07  0:00:07 --:--:--  522M

[bagder]

Thanks!

(I did two minor edits before I merged that fixed two warnings)