UnixOutputStream implements OutputStream for writing to a UNIX file descriptor, including asynchronous operations.
(If the file descriptor refers to a socket or pipe, this will use
poll to do asynchronous I/O. If it refers to a regular file, it
will fall back to doing asynchronous I/O in another thread.)
Note that `<gio/gunixoutputstream.h>` belongs to the UNIX-specific GIO interfaces, thus you have to use the `gio-unix-2.0.pc` pkg-config file when using it.