Eliminate the global variable "shutdown_loop".

Remove the const_cast from HttpdClient.cxx, and avoid one allocation
in ClientRead.cxx.

The header glib_compat.h is now obsolete and can be removed.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Reduces header dependencies.

Replaces GLib's GError.

Implement an event loop without GLib.

Fixes regression from commit 018f4155.

Prepare for migrating away from GLib.

Unsigned and portable.

Handle timeout internally.

Simplify the API, don't use GLib specific integer type.

Remove a GLib dependencies from class EventLoop and DatabaseLock.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

Track which thread runs the EventLoop and provide a check whether
we're currently inside.

Prepare for adding more code.

Merge with CommitEventFlags().

Errors are fatal now.  This makes the class a lot easier to use.

Remove the runtime check for eventfd(), hard-code the feature once
it's been selected at compile time.  The class WakeFD is splitted into
EventFD and EventPipe, using WakeFD as a macro diversion.

No caller needs this.  Fixes use-after-free after returning from
Client::OnTimeout().

Fixes assertion failure.