mixer: always restore volume (even with pulse), don't unmute

Be less clever, and restore the volume state even with AOs like pulse,
which have per-application audio.

Before this commit we didn't do this, because the volume is global (even
if per-application), so the volume will persist between invocations. But
to me it looks like always restoring is less tricky and makes for easier
to understand semantics.

Also, don't always unmute on exit. Unmuting was done even with ao_pulse,
and interfered with user expectations (see #1107).

This might annoy some users, because mpv will change the volume all the
time. We will see.

Fixes #1107.