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
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
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
