Avoid NaN to allow -ffast-math.

Use a boolean flag instead.

Avoid NaN to allow -ffast-math.

Avoid NaN to allow -ffast-math.

Fixes crash.

Restores the features from the previous draft commands "findin" /
"searchin".

This reverts commit a577944a.

Will be replaced by new options for the old commands "search" and
"find".

According to the protocol documentation, matching the file name was
wrong.  This removes some awkward special-case code.

Merge lots of duplicate code.

There hasn't been a maintainer for this plugin for years.  It may even
fail to build.

Fixes the OpenAL detection which was broken because enable_osx was
uesd which was set later.

Share the Mutex between the DecoderThread and the PlayerThread.  This
simplifies synchronization between the two threads and fixes a freeze
problem: while the PlayerThread waits for the DeocderThread, it cannot
answer requests from the main thread, and the main thread will block
until the DecoderThread finishes.