Rename IPPROTO 33 from SEP to DCCP

IPPROTO 33 is DCCP in the IANA Registry:
https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml

IPPROTO_SEP was added about 20 years ago in r33804. The entries were added
straight from RFC1700, without regard to whether they were used.

The reference in RFC1700 for SEP is '[JC120] <mystery contact>', this is an
indication that the protocol number was probably in use in a private network.

As RFC1700 is no longer the authoritative list of internet numbers and that
IANA assinged 33 to DCCP in RFC4340, change the header to the actual
authoritative source.

Reviewed by: Richard Scheffenegger, bz
Approved by: bz (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21178