New dot command .probe:

Code to enable current measurements at all top level
device pins and to enable differential voltage measurements
between two (arbitrary, top level) device nodes.

Adds 0 V voltage sources for current measurements to selected or all
device nodes at top level (including X lines)

Adds an E source excited by two arbitrary nodes for differential
potential (aka voltage) measurements between these nodes

Function inp_probe() is called from inpcom.c