Declare absolute-volume-alist and instrument-equalizer-alist as session-local

Declare absolute-volume-alist and instrument-equalizer-alist as session-local

At least instrument-equalizer-alist is advertised as sort of a
user-settable variable, and while we ask users to tamper with it, we
might as well restore it at the beginning of next session.  This does
not really replace a proper user interface instead of the pathetic
excuse we have for it now.

There is a regtest input/regression/midi-volume-equaliser.ly which is
supposed to demonstrate this "feature" but does not actually dare
setting a value for "flute" equalisation that would differ from the
default value, probably in order not to mess with other
files/regtests.  So the regtest is actually carefully designed not to
test anything.

But that's a different issue.  At least it will be once this patch is
integrated.