configure.ac: prepare for 0.18.15
thread/Posix{Cond,Mutex}: don't ues PTHREAD_*_INITIALIZER on NetBSD

On NetBSD, PTHREAD_MUTEX_INITIALIZER and PTHREAD_COND_INITIALIZER are
not compatible with C++11 "constexpr" (see Mantis ticket 0004110).  As
a workaround, don't ues "constexpr", and use the functions
pthread_mutex_init(), pthread_mutex_destroy(), pthread_cond_init() and
pthread_cond_destroy() instead.  This adds some runtime overhead, but
is portable to POSIX implementations that have awkward initializer
macros.
command/list: reset used size after the list has been processed
OutputThread: retain negative mix ratio

Fixes MixRamp breakage.
test/test_protocol: add missing unistd.h include
Merge branch 'v0.18.x'
tag/ApeReplayGain, decoder/mad: move duplicate code to tag/ReplayGain.cxx
decoder/{vorbis,flac}: move duplicate code to tag/VorbisComment.cxx
tag/ReplayGain: move code to template function
tag/ReplayGain: add VorbisComment parser

Move code from the Vorbis and FLAC decoder plugins.
decoder/flac: pass VorbisComment to flac_parse_replay_gain()
decoder/mad: move code to tag/MixRamp.cxx
decoder/flac: move code to tag/MixRamp.cxx
decoder/flac: pass VorbisComment to flac_parse_mixramp()
decoder/mpg123: make variables more local
decoder/mpg123: support ID3v2, ReplayGain and MixRamp