Since we switched from autotools to Meson in commit
94592c1406, 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.
The normal I/O event thread can have a large latency, e.g. when
libgnutls loads all TLS CA certificates for a https connect. This
makes it unreliable for the ALSA I/O notifications, and causes ring
buffer xruns. To avoid interfering with high latency events such as
CURL's, we move the ALSA I/O events to a separate I/O thread which
also obtains real-time scheduling (if possible).
Closes#221
