Move code from the Vorbis and FLAC decoder plugins.

Eliminates the functions _new() and _free().

Support these PCM formats natively, instead of letting libsndfile
convert everything to 32 bit.

Prepare for other sample formats.

Workaround for assertion failure in PrintUniqueTag().

Fixes MixRamp breakage.

Break the mainloop and terminate run call.

The second time init was called, http_200_aliases pointed to a freed pointer
and leaded to a SEGFAULT.

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.