configure.ac: prepare for 0.18.10
decoder/gme: fix memory leak in container_scan()
decoder/ffmpeg: pass AVSEEK_FLAG_ANY to av_seek_frame()

This corrects a major mistake from commit 724a59aa - there was one
small thing that commit was supposed to do, and it failed.
AV_TIME_BASE is not a seek flag.
decoder/ffmpeg: handle unknown stream start time
IOThread: make io_thread_get() "const"
icu/Collate: fix memory leak in IcuCaseFold()
PlaylistEdit: don't interrupt playback when current song gets deleted
decoder/sndfile: work around libsndfile bug on partial read
release v0.18.10
Merge tag 'release-0.18.10'
input/nfs: add missing string.h include

For strrchr().  Fixes build failure.
DatabaseCommands: clarify compatibility comment
DatabasePrint: convert "type" to unsigned
DatabaseCommands: fix crash on "list base"

The string "base" is understood by locate_parse_type(), but not by
listAllUniqueTags().  The special tag type LOCATE_TAG_BASE_TYPE causes
a crash in PrintUniqueTag().
doc/protocol.xml: update and fix command "list"
doc/protocol.xml: change "in" to "base"
util/{Const,Writable}Buffer: add typedef reference_type
util/{Const,Writable}Buffer: add front(), back(), pop_{front,back}(), shift()
SongFilter: convert argv to ConstBuffer
db/Helpers: use reference instead of pointer