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.

Some, or most, Win32 audio players will pass "icy-metadata:1" to
streaming servers. MPD expects a space between ":" and "1" at this
point, and thus does not send any stream metadata. This applies to
foobar2k and Winamp, for example.

According to forums.radiotoolbox.com/viewtopic.php?t=74 not having
a space there is expected behavior, so maybe MPD could accept
both forms to determine if metadata should be sent or not.

Implements Mantis ticket 3843.

Check was the wrong way around, and made all valid values invisible.

Don't require a writable string, and don't modify it.