Fix breakage from commit bea5681f

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

This commit is similar to 788e3b31,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

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

Will move attributes from struct AudioOutput that are specific to the
OutputThread.  The new struct AudioOutputControl is a holder for the
AudioOutput pointer.

This prepares for making the output list more dynamic, to allow moving
outputs to between partitions.

Some missing bits when converted to C++ exceptions...

Yet another potential crash bug fix.

Fixes potential crash bug.

Doesn't make a practical difference - but it's more correct this way.

Changed AlsaMixerPlugin to use the get and set normalized functions from volume_mapping of alsa-utils/alsamixer
Changed volume_mapping set volume to be for all channels and not per channel
added volume_mapping files to Makefile.am

source:
http://git.alsa-project.org/?p=alsa-utils.git;a=blob_plain;f=alsamixer/volume_mapping.c;hb=HEAD
http://git.alsa-project.org/?p=alsa-utils.git;a=blob_plain;f=alsamixer/volume_mapping.h;hb=HEAD

Make all the locks exception-safe.

Also uses the notification system to display tags.

The old struct config_param remains only for top-level string options.