CVE-2017-12986/IPv6 R.H.: Update to reflect the actual IPv6 RFC.

In RFC 1883, the Type 0 routing header had a 1-byte reserved field and a
3-byte strict/loose bit map; in RFC 2460, that changed to a 4-byte
reserved field.

This fixes a buffer over-read discovered by Brian 'geeknik' Carpenter (by
making an ND_TCHECK() call check for the presence in the captured data
of all 4 bytes of the reserved field; we were printing it as a 4-byte
field, so we needed to check for them).

Add a test using the capture file supplied by the reporter(s).