top: fix sorting by pid (-o pid)

The 'pid' order was added, but the equivalent comparator was not back in
8b800d3af2d8c ("Add the PID column to the list of sort keys.").
ordernames indices need to match compares indices in order for this
to work.

A future commit will restructure all of this to make it less error prone
going forward (see D37083).

Reviewed by: pstef (D37083)
Approved by: re (cperciva)

(cherry picked from commit deb63adf945d446ed91a9d84124c71f15ae571d1)
(cherry picked from commit cfd3bd04c245889d339c4f7cb1e45c955537e3eb)