Use FNONBLOCK instead of SS_NBIO to check/indicate that the I/O mode

Use FNONBLOCK instead of SS_NBIO to check/indicate that the I/O mode
for sockets is non-blocking.

This allows us to G/C SS_NBIO.  Having to keep the two flags in sync
in a mp-safe way is complicated.

This change introduce a behavior change in sosplice(), it can now
always block.  However this should not matter much due to the socket
lock being taken beforhand.

ok bluhm@, benno@, visa@