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

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