As analyzed in #12678 there are cases where different reports can be
generated for the different snips of the packet send via the `sock`.
To catch all errors generated by the stack, the sock has to subscribe
for all snips of the packet sent. If any of the snips reports an error
distinct from `GNRC_NETERR_SUCCESS` or the previous one, we report that
status instead of just the first we receive. This way we are ensured to
have the first error reported by the stack for the given packet.
sock_[udp|ip]_recv returns `pkt->size` after pkt was released via `gnrc_pktbuf_release(pkt)`.
This can result in wrong values returned by this functions and thus is not according to its sepecification.
Storing this values before releasing pkt returning the stored values should fix this.
gnrc_sock_recv used to duplicate functionality of gnrc_ipv6_get_header,
but additionally checked whether the IPv6 snip is large enough.
All checks are now included in gnrc_ipv6_get_header, but as most of them
stem from programming / user errors, they were moved into asserts; this
constitutes an API change.
gnrc_sock currently does not compile with `gnrc_neterr` included, since
both `msg_try_receive()` and `msg_try_send()` expect a *pointer* to a
`msg_t`, not a `msg_t`.
To test, just compile an application using `gnrc_sock` (e.g.
`examples/gcoap`) with `USEMODULE+=gnrc_neterr`.
