Add some more parentheses, Just In Case.

I'm not sure whether

	Performing a byte swapping operation on "p" implies that it came
	from an external source, and is therefore tainted.

from Coverity means that it thinks we're byte-swapping the pointer
*itself*, or that we're byte-swapping what it points to, but, just in
case it's the former, let's try throwing some more parentheses in.

(If it's the latter, well, yes, it's packet data, so it comes from an
external source, but Coverity didn't seem to point out any place where
we were using the data it points to without checking its value in cases
where we have to.)
Leave it up to ip6_print() to handle non-IPv6-capable systems.

Always define and declare ip6_print(), always compile print-ip6.c, and
always call it if we recognize a payload as IPv6.  If INET6 isn't
defined, ip6_print() will just print the length and note that printing
isn't supported.

That way, we don't do weird dissection of IPv6 packets on systems
without IPv6 support, due to, for example, ethertype_print() returning 0
("not dissected") for IPv6 packets on those systems (IPv6-over-Frame
Relay was dissected weirdly due to this).
Fix indentation.