This introduces a new alternative and better API to `conn`. It differs in the
following aspects:

* a common address type for both IPv4 and IPv6 addresses is introduced
* communication end-points are abstracted as end-point types `sock_x_ep_t`,
  containing the address, its family, its port (if required for protocol) and
  the interface identifier.
* All functions require some kind of state. Sending of datagrams to the same
  source or replying to incoming datagrams is thus simplified
* TCP connection establishment was overall reworked: connected sockets and
  listening sockets are now two distinct types. An accept on a listening socket
  than yields a connected socket

Fix wrongly typedef'd endpoint

Don't use netif parameter for static IP end-point initializers

Reword confusing out-parameter doc in create functions

Fix some typedefs

Simplify static intializers.

Fix inheritence of _sock_tl_ep.

Add `-ECONNABORT` return for sock_tcp_read and sock_tcp_write.

sock_tcp: more error code specifications and fixes

sock_ip: Provide proto parameter to send in case sock == NULL.

Fix some typos

Include missing header

Add more error classes