IP2BIN, IP2HBIN, IP2STR: refactor `interp` call-backs

IP2BIN, IP2HBIN, IP2STR: refactor `interp` call-backs

IP2STR and IP2BIN do all family checks inside the for-loop that converts the
address fields, whereas IP2HBIN does the checks once before the loop.  Refactor
the former to do as the latter.

Also, move all the remaining contents of the for-loops, apart from the
`pp_is_valid` checks, into `ip2*` functions.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>