This is not needed. And for the normal mac builds, we even do not
have this library at all.

close #13838

There are still many cases where waking up the thread sleeping is only
half of the problem and waking it up half of the solution. For instance,
the net_* and stream_* I/O functions have no ways to return a non-fatal
error.

This reduces the system call overhead.

It was always either true or unused. Note that when the buffer length
is 1 byte, waitall had no effects.

net_Read() and net_Write() are now symmetric.

This saves one file descriptor and some kernel resources.

This is a generic and inefficient but functional implementation.