The use of TCP_ in udp.h is not known, fortunately udp.h is not
specified by posix so there are no strict namespace rules, added in
linux commit e27cca96cd68fa2c6814c90f9a1cfd36bb68c593
xfrm: add espintcp (RFC 8229)
add UDP_NO_CHECK6_* to restrict zero UDP6 checksums, new in linux commit
1c19448c9ba6545b80ded18488a64a7f3d8e6998 (pre-v4.18 change, was missed)
add UDP_SEGMENT to support generic segmentation offload for udp datagrams,
bec1f6f697362c5bc635dacd7ac8499d0a10a4e7 (new in v4.18)
the affected part of the header is responsible for providing both GNU
and BSD versions of the udphdr structure. previously, the
namespace-polluting GNU names were always used for the actual struct
members, and the BSD names, which are named in a manner resembling a
sane namespace, were always macros defined to expand to the GNU names.
now, unless _GNU_SOURCE is defined, the BSD names are used as the
actual structure members, and the macros and GNU names only come into
play when the application requests them.
