When the client wants to seek, but the decoder has already finished
decoding the current song, the player restarts the decoder with an
initial seek at the new position. When this initial seek fails, MPD
pretends nothing has happened and plays this song from the start.
With this new flag, a restarted decoder marks the initial seek as
"essential" and fails the decoder if that seek fails.
Closes https://github.com/MusicPlayerDaemon/MPD/issues/895
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.
