unix: avoid duplicate recvmmsg terminal callback (#5133)

Stop `uv__udp_recvmmsg()` from emitting its own callback when
`recvmmsg()` returns `<= 0`, since this duplicates the handing in
`uv__udp_recvmsg()` which emits the same callback again.

Add a regression test that keeps the socket open past
`UV_UDP_MMSG_FREE` and fails on a second terminal callback.

Co-authored-by: Isaac Elbaz <587490+script3r@users.noreply.github.com>