pg_type questions

I'm working on psql printing routines here and want to do alignment by
datatype. Two questions arose:

1) What is the difference between/merit of "line" vs. "_line", "cidr" vs.
"_cidr", etc.? Do I have to worry about them?

2) Can I assume that the Oids for the datatypes are always the same
(barring a developer changing them, of course)? Where are they defined?
What would be the best way to digest the output of libpq's PQftype()?
(Perhaps a char * PQftypetext() would be of general use?)

(The issue here is that I do _not_ want to have to query pg_type for that
information, since psql has no business contacting the database server
when not asked to do so.)

Thanks,Peter

-- 
Peter Eisentraut                  Sernanders vaeg 10:115
peter_e@gmx.net                   75262 Uppsala
http://yi.org/peter-e/            Sweden