socket/netconn recv: FIN should only be reported once

FIN should only be reported once (as '0' for sockets, as 'ERR_CLSD' for
netconns). Before this change, ERR_CLSD was returned forever...

This is the 2nd try. First try (commit ebcae98ae65c26a0c210c802540bf027d07fe2f1)
was buggy in that it could drop the FIN if it was read together with data
(reverted in commit ebcae98ae65c26a0c210c802540bf027d07fe2f1).

This version fixes this by adding an apiflag and a netconn flag to keep
track of this.

socket unit test: check receiving FIN together with data works

test_sockets: disable test for calling recv() after remote closure

Re-enable these when calling recv() multiple times after remote closure
returns an error

Fix broken MSG_PEEK on TCP sockets (post-2.0.3 bug)

MSG_PEEK on TCP sockets was broken since commit b71d4477eab7c19c34e57e5c9a1f8add63de7879
from 06.03.2017: recv hung in an endless loop and tcp_recved() was called for peeked data
(which would result in a too large window advertised).

Aded TCP MSG_PEEK to socket unit tests

socket unit tests: improve basic tests a bit

Finally fix bug #50088 (socket/netconn: data before RST should be readable) and added a unit test for it

Ensure that unit tests leave the stack in a clean state

Moved NUM_SOCKETS and struct lwip_sock to sockets_priv.h; added test case for fd_use count with select

fix test_sockets_msgapi_cmsg unit test (msvc gives me "warning C4706: assignment within conditional expression")

test_sockets: fix pointer to integer conversion

This fixes a pointer to integer conversion in test_sockets_msgapi_cmsg()

This was identified by Travis CI: https://travis-ci.org/yarrick/lwip-merged/builds/225640702

sockets: task #14247, add CMSG and IP_PKTINFO

This commit adds CMSG infrastructure (currently used with recvmsg) and
the IP_PKTINFO socket option.

In order to use IP_PKTINFO, set LWIP_NETBUF_RECVINFO to 1

Unit test is added to verify this feature

Patch #9307: Replace mem_malloc+memset with mem_calloc

Aside from reducing source code, on systems which use MEM_LIBC_MALLOC,
this has the potential to improve performance depending on the underlying
memory allocator

See http://stackoverflow.com/questions/2688466/why-mallocmemset-is-slower-than-calloc

lwip_fcntl() returns access modes

test_socket: conditional compile buffer trailer code

This code is marked as dead when BUF_SZ is a multiple of 4 (current
situation with unit tests)

This hopefully fixes a -Wunreachable-code failure found by Travis CI

test_socket: add sendmsg/recvmsg TCP test

This migrates the sendmsg TCP test from socket examples (task #14408)
to socket unit tests

Additionally, this adds support for testing recvmsg, creating a TCP
test for both sendmsg/recvmsg (referred to as msgapi test)

This also makes a small change to msgapi UDP to clear the receive
buffer after verifying the previous datagram

test_sockets: move loopback addr logic to utility function

This moves the loopback address creation logic to a utility function
so it can be shared with forth coming message API TCP test

test_socket: replace {0} with memset

Using {0} broke Travis CI even though this should be correct for
initializing struct msghdr (see example in Linxu man pages:
http://man7.org/linux/man-pages/man3/cmsg.3.html)

Just use memset for now which is the common approach in LwIP codebase

test_socket: convert sendmsg test to use recvmsg

This converts the sendmsg test to use recvmsg for receiving, thus
exercising both sendmsg and recvmsg in a single test

This also adjusts the test naming to communicate all message APIs
(sendmsg/recvmsg) are being tested

test_sockets: fix warning about unused variable "addr_size"

sockets unit tests: removed commented-out test code :-/

tried to fix sockets unit tests; fix configuration to run with any NO_SYS setting and with/without IPv6 (IPv4 is required)

task #14408: move sendmsg UDP to unit tests

This commit moves the sendmsg UDP test from socket examples to socket
unit tests

The test has been converted to send/receive on the loopback interface
and also test a connected sendmsg with NULL msg_name

Fix and improve sockets unit test and unit test sys_arch (with a little help of tcpip.c)

tried to add basic socket unit tests (nonsense only for now); made LOCK_TCPIP_CORE()/UNLOCK_TCPIP_CORE() overridable for that