configure.ac: prepare for 0.18.21
decoder/ffmpeg: detect and fix negative time stamps

Works around assertion failure due to something that appears to be a
(minor) FFmpeg bug.
playlist/embcue: fix filename suffix detection

The definition of the playlist_plugin struct member of the embcue
plugin was incorrect.
util/HugeAllocator: enable MEM_COMMIT on Windows

Without MEM_COMMIT, the reserved address space is not accessible, and
MPD crashes.
LogInit: make stderr line-buffered

Make sure everything gets logged right away.  No delays because
stdio's buffer is not yet full.
LogBackend: force-flush stderr on WIN32

setvbuf() does not seem to have an effect on Windows.
release v0.18.21
Merge tag 'v0.18.21' into v0.19.x
release v0.19.7