Add new 'dfwalk' utility to do recursive 'readdir'

Work in progress, fix up the commit message and utility description in
the usage function. Should probably also add a simple man page. Also,
add more commentary on usage/examples in this commit message.

This patch adds a simple utility to do a recursive 'readdir' on a given
directory that is passed as an argument.

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