Various improvements to dbufstat.py

 * Retab'd the file to use 4 spaces for indentation (to be consistent
   with arcstat.py)
 * Modified the internals to be more resilient to column reorders and/or
   new column additions
 * Updated column labels to be unique
 * Implemented the "-r" and "-rr" options
 * Implemented the "--buffers"/"-b" option
 * Plus other various "improvements" to the internals of the script

Signed-off-by: Prakash Surya <surya1@llnl.gov>