File descriptor sending support via new `sendfd:` scheme
This MR adds support for sending file descriptors between processes via a new kernel scheme,
sendfd:. Processes expecting to receive fds will open
O_CREAT set and read or wait for events on this fd. Processes wanting to send fds will open
O_CREAT clear and simply write the fd as bytes into the socket. The receiving end will then read a
syscall::data::ReceivedFd struct (see this MR on syscall) from the socket which contains the new fd number and the PID of the sending process.
I haven't been able to test this yet.