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".