Use the nd_uintN_t types more.

Use the nd_uintN_t types more.

Define them in netdissect.h.

Use them in ip.h, and *don't* mark the structures as UNALIGNED; that
should no longer be necessary.

Add a new nd_ipv4 type to use as an IPv4 address; it represents the 4
bytes of IPv4 address as an array of unsigned chars, so that its natural
alignment is only on a byte boundary.

Those changes found some places where we weren't using
UNALIGNED_MEMCPY() to extract IPv4 addresses from packets; use it.