libnv: read entire datagram in nvlist_recv()

[rew]

Summary

When SOCK_DGRAM is used, a portion of the datagram is discarded during
the initial recv() when getting the nvlist_header.

To workaround this, use MSG_PEEK for the initial recv() when using a
datagram socket.

[oshogbo]

Could you also add test for this case?

[rew]

add ATF tests for SOCK_DGRAM with nvlist_send()/nvlist_recv().

The test-case nvlist_send_recv__send_many_fds_dgram, wouldn't pass
without sysctl tuning. Specifically, net.local.dgram.maxdgram and
net.local.dgram.recvspace.

Not sure if this test should be enabled by default.

[rew]

remove blank line

[oshogbo]

@lwhsu Is those settings ok with our tests machines?

[rew]

ping @lwhsu

Let me know if there's anything I can do to help to get these tests in.

If the settings are unacceptable for the test machines, I can atf_skip the offending test.

Just to be clear, I've added 3 tests - and only the one test-case needs sysctl tuning to pass.

[rew]

Any objections here?

If I don't hear back from @lwhsu , I'll disable the nvlist_send_recv__send_many_fds__dgram test case on commit.

[oshogbo]

Feel free to commit this.