Pass struct sockaddr to ngtcp2 so that it knows how to deal with
address change in more detailed manner (e.g., just compare IP address
without port).

This commit adds special handling for the very first Initial packet
which is corrupted and discarded.  If server receives such packet, it
has to drop connection silently without sending any CONNECTION_CLOSE.
Then valid packet might come later.  Keeping connection state is
harmful because corrupted packet has wrong SCID, which drops later
valid packet because mismatched SCID.  New error code
NGTCP2_ERR_DROP_CONN is added which tells server application to drop
connection silently.

Sending CONNECTION_CLOSE with exponential backoff will be implemented
in a later commit.

This patch represents phase 2 of the "Close all
connections on exit" feature, as well as including
a number of other benefits for both client and server:

- Packet writing is now moved to a function: send_packet
  this function will sense a number of different errors
  on the sendto system call, and categorise these as
  either fatal or non fatal (able to be retried).
  Non fatal errors will most likely be caused by the
  kernel tx queue being full, which means the packet
  should be writable at a later time.
  This code is loosely based on similar code in the
  Berkeley Internet Name Daemon (BIND), which I
  assume would know how to send UDP packets.
- Functions that write packets will observe the
  send_packet return value and if the network error is
  non fatal, the packet that was to be sent will be
  retained and writability on the socket file
  descriptor will be polled through the event reactor.
- When writes are once again possible on the socket,
  the stored packets will be sent first, to preserve
  ordering at the send side, and only once this
  packet is sent will the core library be queried
  for more new transport level and application level
  packets, which themselves may need to be queued.

At the moment, it just extracts Largest Acknowledged.  It is rather
implemented as place holder to parse incoming ACK frames without
error.