pf: simplify pf_addrcpy() and pf_match_addr()

Use the v4/v6 union members rather than the uint32_t ones.
Export IN_ARE_MASKED_ADDR_EQUAL() in in_var.h and use it (and its IPv6
equivalent) for masked comparisons rather than hand-rolled code.

Event: Kitchener-Waterloo Hackathon 202406