First in a series of changes to leverage on recently introduced

First in a series of changes to leverage on recently introduced
hierarchical vimage naming model in the kernel.

The main thrust of this change is to decouple experiment id ($eid)
from vimage / netgraph node names.  Instead, a top-level vimage
named $eid is created first, in which all vimages / netgraph nodes
are created as child nodes.  This relieves the default vimage
context of any netgraph space pollution, and paves a way for
adding support for parallel execution of independent experiments.

This change does NOT include a revamp of experiment cleanup
routines, so be warned that until further notice neither "imunes -b"
nor attempts to stop an experiment from the GUI will work.

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