libnss-resolve: Don't duplicate "dns" query

debian/libnss-resolve.postinst: Replace "dns" entry with "resolve", as
libnss-resolve already falls back to dns if resolved is not available. This
avoids querying failing DNS twice. Conversely, replace "resolv" with "dns" in
postrm.

Also fix the unsafe substitution in postrm: We want to match on full words,
thus use \b to match word boundaries. \Q and \E have no effect as none of the
characters in "resolve" are special regex characters.