Closes pkg/10535. Changes are:
o Bypassed the hard-to-reproduce build failure caused by
getrusage() returning decreasing microsecond times. Did
this by wrapping the getrusage() function so that it never
returns a smaller value for microseconds than the previous
time it was called, if called within the same second.
Perhaps this should be done to getrusage() internally, until
fixed the proper way. See PR bin/10201. --- refling.
o Added a tutorial and a message about it. --- refling.
