top: fixup logic for Nehalem cpu 'idle' threshold

top: fixup logic for Nehalem cpu 'idle' threshold

Due to a poorly constructed temporary fprintf
used during development, an earlier commit went
a little too far in its computations.  The net
result was the code looked nice but actually
accomplished nothing.

It is the /proc/stat line 1 (summary line)
whose tics must be used in establishing the
threshold boundary.  And that calculation
need be performed just once per frame.

This commit ensures one threshold calculation
per delay interval no matter how many cpus
are ultimately displayed.

It also corrects scalability by factoring in
the total number of online processors.

Reference:
commit 9e7dd43ab7b2e68177cefd49b3c07311ea79bc1a