Commit 4d6a8663fa0932a8c1b40639464f68af670f0808 made pipes and
named pipes block when opening for O_WRONLY or O_RDONLY. Local
sockets, however, require `local_open_client_tx` to be non-blocking
to enable the server side to prevent the server side from blocking.
If set otherwise, it would deadly block. This commit sets the FIFO
as non-blocking temporarily, open the TX side and, if originally
blocking - restores it to that state.
4f4a00ab1f