Connect minish-once to minish using a pipe

Connect minish-once to minish using a pipe

The pipe is created by minish.
minish uses fcntl() to set the O_NONBLOCK flag on the pipe,
so that read() returns -1 instead of blocking when the pipe is empty.
The writing end is passed to minish-once
by writing the bytes of the file descriptor as arguments,
one byte per argument.
If the byte is not 0, it is followed by 0.
Before terminating, minish-once writes its return code in the pipe;
minish reads it and terminates itself with the same error code.
minish-once closes its end of the pipe before attempting execv(),
so that the executing command has no way to access it.
If execv() fails, minish-once does not write in the pipe,
because minish should not terminate in that case.