Fixes build failure on OS X, closes #44.  With the other plugins,
that's not critical, because those use the AudioOutputWrapper, which
hides this problem.

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fixes another buffer overflow: if the stream has a very long title or
URL, resulting in a metadata string of more than 2 kB, icy_string[0]
is a negative value, which gets casted to size_t - ouch!

 https://bugs.musicpd.org/view.php?id=4652

Fixes a buffer overflow due to the bad formula rounding the buffer
size up.  At the same time, remove the "+1" from the meta_length
calculation, which takes the padding into account and at the same time
implements proper rounding.

Why did this cast exist??

Same as in commit e02d8ad8, but this time for the sndio plugin
which can be emulated by libroar.

I want a compiler warning when a new state needs to be considered
here.

Fixes the dreaded error "Failed to create : No such file or
directory".

 https://bugs.musicpd.org/view.php?id=4625

We'll have some code for it soon.

Move this sample rate fixup from the ALSA output plugin to PcmExport,
where it belongs.

Prepare for DSD sample rate fixups.

Beware, this commit was not tested.  I don't have OS X, but I want to
prepare an API change.

Eliminate all those "goto"s and make the function exception-safe.

Beware, this commit was not tested.  I don't have Haiku, but I want to
prepare an API change.

Avoid one level of dynamic allocation.