Add header files for struct and accessors for IPv4, UDP, and TCP.
Only parts of the fields of the structs have been seen while working
on wireless drivers. The remaining field names are filled up with
the FreeBSD field names for now. If you have insights into their
correct naming in Linux, feel free to adjust.
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30428
