... and make TimerEvent a type alias for FineTimerEvent (i.e. swap
names).

Time to move on, two years after 2017.

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.

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

Prepare to eliminate the TimerRecord struct.

Now that the remaining known bugs in poll() implementation are fixed,
we can go on without the GLib implementation.

This change adds two configuration options:

  --with-eventloop=[glib|internal|auto]
  --with-pollmethod=[epoll|auto]

First allows switching between GLib event loop and internal one.
Second chooses backend to use for internal event loop.
Conditional compilation symbols are changed accordingly.
Additional helper macro MPD_OPTIONAL_FUNC_NODEF is added as well.

Implement an event loop without GLib.

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