The method control() is too complicated, and overengineered.  Replace
it with two trivial functions: get_volume() and set_volume().

The mixer plugins should re-use the mixer struct and incorporate it in
their object class.

Both methods are always called together.  There is no point in having
them separate.  This simplifies the code, because the old configure()
method could be called more than once, and had to free old
allocations.

Allocate the mixer object when it is configured.

Merged mixer_configure() into mixer_new().  mixer_new() was quite
useless anyway.

Don't use statically allocated mixer objects.

The plugin structures must never be modified.

All config_get_block_*() functions should accept constant config_param
pointers.

Renamed functions, types, variables.

This patch tryes to introduce pluggable mixer (struct mixer_plugin) along with some basic infrastructure (mixer_* functions). Instance of mixer (struct mixer) is used in
alsa and oss output plugin