rgw/rgw-orphan-list: refactor / add error checking

rgw/rgw-orphan-list: refactor / add error checking

standardize log output
add check that supplied pool[s] exist on the cluster
add return code checks for all commands that can affect output
add check for empty intermediate files
add usage output, including a -h command line option
add command line option to specify temporary directory to use
  ( instead of /tmp )
require command line supplied pools to be encased in double quotes "
  when multiple pools are provided
add warning for indexless buckets
move major segments to separate functions

Signed-off-by: Michael J. Kidd <linuxkidd@gmail.com>