Yet another potential crash bug fix.

Fixes build breakage by commit 4e5271fc; and this method does make
sense in non-debug builds.

Fixes potential crash bug.

Now ClearSocketList() may only be called from PrepareSockets().
Calling it before destroying the object doesn't work properly, because
it doesn't unregister the TimeoutMonitor and the IdleMonitor.  Some of
its callers need to be fixed.

Change EventLoop::IsInside() call to EventLoop::IsInsideOrNull().
This means that BlockingCall() may be used during shutdown, after the
main EventLoop::Run() has finished.  This is important because mixers
are currently registered in the main EventLoop.

The libsidplayfp fork has merged libsidutils into the main library.
The libsidutils we used to link with was part of the original
libsidplay project.

Doesn't make a practical difference - but it's more correct this way.

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

Same as in commit e02d8ad8, but this time for the sndio plugin
which can be emulated by libroar.

GCC 4.9 has incomplete C++14 support.  Specifically, it doesn't allow
switch/case in "constexpr" functions.

With "ok==false", newly created threads may quit instantly.

libupnp provides seconds, not whatever time unit is used by
std::chrono::steady_clock.