perlipc: modernize open and handle usage

perlipc: modernize open and handle usage

Replace bareword filehandles with lexicals, use three-arg open wherever
possible, explain that two-arg open may still be necessary for piped opens on
non-Unixy platforms, separate the discussion of three-arg pipe open from list
open, pass the now-lexical filehandles into the spawn function in the TCP
examples, and remove the obsolete explanation of lexical filehandles.