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.
db/proxy: support empty values in VisitUniqueTags()

Workaround for assertion failure in PrintUniqueTag().
decoder/sndfile: add sndfile_sample_format()

Prepare for other sample formats.
decoder/sndfile: move sf_readf_int() call to sndfile_read_frames()
decoder/sndfile: support float and 16 bit samples

Support these PCM formats natively, instead of letting libsndfile
convert everything to 32 bit.
decoder/sndfile: log detailed sf_open_virtual() error message
doc/user: more NFS instructions
doc/user: add section "Satellite setup"
decoder/sndfile: remove debug printf()
decoder/sndfile: remove obsolete code comment
decoder/audiofile: fix indent
decoder/sndfile: include cleanup
decoder/adplug: move the buffer into the loop
decoder/adplug: log version number
DecoderBuffer: use class DynamicFifoBuffer
DecoderBuffer: convert pointer to reference
DecoderBuffer: make "decoder" const