Add "-T nodetype" option to only gather alloc/free stats for one node type.
Add "-T nodetype" option to only gather alloc/free stats for one node type.

Also, changed the verbose summary output to include totals for all listed nodes.
I used both of these changes to gather "node hour" usage for d820 nodes.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome