This makes the tool consistent with the option set of oid2name, which
has been historically using -f for filenodes, and has more recently
gained long options and --filenode via 1aaf532d.

Reported-by: Peter Eisentraut
Author: Fabien Coelho
Discussion: https://postgr.es/m/97045260-fb9e-e145-a950-cf7d28c4eaea@2ndquadrant.com