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.

Those have been superseded by the new legacy configuration code.

The plugin structures must never be modified.

Return the default value in the conf_get_block_*() functions when
param==NULL was passed.

This simplifies a lot of code, because all initialization can be done
in one code path, regardless whether configuration is present.

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

This replaces lots of getBlockParam() invocations.

The g_free() function includes a NULL check.  We don't have to do it
twice.

Renamed functions, types, variables.

When MPD starts without audio output configuration, the "param"
variable is NULL.  This triggers a segmentation fault in both mixer
plugins.

removing mixer_reconfigure memmory leak, fixing configure of alsa and oss mixer (passing parameters)

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

There is no reason to use the shortcut "mix" instead of "mixer".