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>
Reviewed by: melifaro
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25389