Default to /proc/cpuinfo for maxTaskPar.prediff core counting.

Instead of explicitly calling out the configurations for which we use
/proc/cpuinfo to determine the core count, default to /proc/cpuinfo and
arrange for failure (by putting a '0' in the .good) when it's not there.

Credit: Brad.
Add a comment describing how we're using the contents of /proc/cpuinfo.
Merge pull request #603 from gbtitus/update-maxTaskPar

Default to /proc/cpuinfo for maxTaskPar.prediff core counting.

Instead of explicitly calling out the configurations for which we use
/proc/cpuinfo to determine the core count, default to /proc/cpuinfo and
arrange for failure (by putting a '0' in the .good) when it's not there.
While doing so, be more careful about how we interpret the contents
of /proc/cpuinfo, and document what we're looking for in it.

Reviewed by Brad.