[Nauty] minor release of nauty+traces

[Brendan McKay]

Version 2.6r2 is available at
        http://pallini.di.uniroma1.it
and
        http://cs.anu.edu.au/~bdm/nauty/

Changes:
* Update of configure script.
* Small changes to the manual.
* Fixes to minor problems noted by Keith Briggs and Jernej Azarija

Brendan.