Add a few features to rcorder:

o Enhance dependency loop logging: print full chain instead of the
  last link competing the loop;
o Add -g option to generate dependency graph suitable for GraphViz
  visualization, loops and other graph generation issues are highlighted
  automatically;
o Add -p option that enables grouping items that can be processed in
  parallel.

Submitted by:	Boris Lytochkin <lytboris at gmail>
Differential Revision:	https://reviews.freebsd.org/D25389