[Groff] getting .na, .nh thruout doc for nroff

[Dorai Sitaram]

I'd like to avoid adjustment (justification) and hyphenation on a document
but only if it's being processed by nroff.if n).  In essence, 

.if n .na
.if n .nh

should hold for every paragraph.  What's a robust way to do this?  Trying
to adding this as a hook (via .am) to various things, like the header
macros, or even par*start, always seems to miss the first couple
paragraphs.

Thanks.