Add support for multiple Glaze wrappers to be used at the same time.

Add support for multiple Glaze wrappers to be used at the same time.

We simply use a colon-separated specification of the wrappers in
GLAZE_WRAPPER and hold on to an array of dlopened handles.

It's a simple matter to use the array of handles in several places.

The one place that involves some delicacy is glaze_lookup. Here we
must ensure that one wrapper's wrapped implementation of a function
chains to the subsequent wrapper and not to itself (which would lead
to infinite recursion). To get this right, we inspect the backtrace
and refuse to let glaze_lookup return a function that already appears
in the backtrace.