Folded elvind, plab-adapted evproxy, and program-agent into plab startup.

rc.progagent: Do run on plab, and do so via the wrapper.

GNUmakefile.in: Add new plab startup scripts to rootball

rc.inplab: hook in the elvin startup script.

pa-wrapper: Sachin's program-agent wrapper for running on plab.  Tries to
            keep the agent connected to the local elvind.

runelvin: Sachin's script to fire up elvin and evproxy on Emulab.  Makes
          sure elvin starts up on an unused port if 2917 happens to be
          in use.