Allows callers to extract the NFS error code.

It indeed is a macro on Windows.

See commit 71f0ed8b

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fixes race condition when epoll_ctl() gets called after the socket has
been closed, which may affect a different socket created by another
thread meanwhile.

When rpc_reconnect_requeue() gets called from inside nfs_service(),
the NfsInputStream can stall completely because the old socket has
been unregistered from epoll automatically, but the new one has never
been registered.  Therefore, nfs_service() will never be called again.

This kludge attempts to detect this condition by checking
nfs_which_events()==POLLOUT.

https://bugs.musicpd.org/view.php?id=4081

Make sure we're using the same EventLoop as the NfsManager.

Eliminate dependency on io_thread_get().

Required for Boost 1.61, which uses that overload in a BOOST_ASSERT().

SocketMonitor::Cancel() does not actually unregister the socket; it
only disables the event.