Pprintast: factorize handling of infix/prefix binders

There was a bug with "external _pipe = ..." in
otherlibs/threads/unix.ml where the identifier _pipe was wrongly
detected as infix and reprinted as "external ( _pipe ) = ...", causing
a syntax error.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14503 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02