Can fail if somebody calls Break().

This replaces the old `dead` flag which was unreliable; it was `false`
if the EventThread was not yet started, which could cause deadlocks in
BlockingCall().

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Fixes thread sanitizer warnings.

Fixes thread sanitizer warnings.

Move its code to DeferEvent instead.

The intrusive contains can easily erase items without searching
through the whole list.  This removes a good amount of runtime
overhead.

By using an "intrusive" data structure, we can easily eliminate
struct TimerRecord.

Prepare to eliminate the TimerRecord struct.

Allows eliminating lots of complexity and workarounds for bogus
assertion failures.

This is only needed while Run() is alive, and moving it there avoids
problems with thread-checking assertions.

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

Kludge for libavahi-client quirk.

Add the debug-only flag "virgin" which gets checked by assert()
calls.  Fixes assertion failures when using zeroconf/avahi.

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.

Add a flag that indicates whether the EventLoop is currently "busy".
As long as that flag is set, it does not need to be woken up - we can
simply add the DeferredMonitor to the list, and it will be caught by
EventLoop very soon.  This eliminates nearly all of the
DeferredMonitor overhead when compared to IdleMonitor, rendering
IdleMonitor mostly obsolete.

Reduce DeferredMonitor overhead.

Improved support for added events.