Fixes regression by commit 28e07e90

Closes https://github.com/MusicPlayerDaemon/MPD/issues/596

Really drain all data from the period_buffer in any case.

This assertion is not about whether all data has been consumed, but
about whether there has been any data at all.

There cannot be partial frames in the buffer, so we don't need
GetPeriodPosition(); it's enough to check whether head has been moved
in this period.

These options are exclusive.

This function is sparsely documented and a look at the bluez-alsa
source code shows that implementations make undocumented assumptions
on the `struct pollfd` array parameter which can lead to strange
effects.

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.

This call was missing, causing very high CPU usage when the ALSA
output plugin was used with dmix.

Closes #391

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

Avoid another potential deadlock: if no file descriptors are
registered, our non-blocking ALSA code cannot ever work.

Allows defining a list of supported audio formats, and allows
switching on and off DoP with certain formats.

This is a first rough draft.  The setting syntax and its semantics may
still be redesigned.

See commit 71f0ed8b