alsamixer: Fix X-win related crash for PulseAudio plugin (bad linking)

alsamixer: Fix X-win related crash for PulseAudio plugin (bad linking)

A lot of reports that alsamixer crashes in X terminal when the PulseAudio
CTL plugin is activated were entered to the tracking systems.

I figured that there is a linking clash for the shutdown() function.
The shutdown() function in glibc is socket related, but the alsamixer code
redefines this function and appearently that under some linking
circumstances - which PA client invokes during the runtime dynamic linking
- the wrong function is called.

This patch, for safety, renames the shutdown() function from alsamixer
to app_shutdown(), but it might make sense to figure the real linking
culprit to avoid future surprises.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>