Adds a file descriptor to the set of file descriptors polled for this context.
This will very seldom be used directly. Instead a typical event source will use add_unix_fd instead.
a MainContext (or null for the default context)
a PollFD structure holding information about a file descriptor to watch.
the priority for this file descriptor which should be the same as the priority used for attach to ensure that the file descriptor is polled whenever the results may be needed.