Support real expressions in pnmcompare.

Unfortunately this is an incompatible change to the command-line
syntax; the expression is after the filenames now, whereas when there
was just one comparison, it was before the files.  I don't see much
alternative, though; perhaps a compatibility heuristic could be added,
to implement the old semantics if there are three arguments, the last
two are files, and the first is one of the old comparison strings.  So
far that doesn't strike me as worth bothering with.