another kernel internal state exposure for checkpoint-restore.
see linux commit b1ed4c4fa9a5ccf325184fd90edc50978ef6e33a

placing the opening brace on the same line as the struct keyword/tag
is the style I prefer and seems to be the prevailing practice in more
recent additions.

these changes were generated by the command:

find include/ arch/*/bits -name '*.h' \
-exec sed -i '/^struct [^;{]*$/{N;s/\n/ /;}' {} +

and subsequently checked by hand to ensure that the regex did not pick
up any false positives.

new fields and associated linux commit:
tcpi_notsent_bytes, tcpi_min_rtt cd9b266095f422267bddbec88f9098b48ea548fc
tcpi_data_segs_in, tcpi_data_segs_out a44d6eacdaf56f74fad699af7f4925a5f5ac0e7f

TCP_CC_INFO is a new socket option to get congestion control info without
netlink (union tcp_cc_info is in linux/inet_diag.h kernel header).
linux commit 6e9250f59ef9efb932c84850cd221f22c2a03c4a

TCP_SAVE_SYN, TCP_SAVED_SYN socket options are for saving and getting the
SYN headers of passive connections in a server application.
linux commit cd8ae85299d54155702a56811b2e035e63064d3d

Add new tcpi_* fields to struct tcp_info implementing RFC4898 counters.
linux commit 2efd055c53c06b7e89c167c98069bab9afce7e59

Programs such as iptables depend on these constants, which can also
be found defined in other libcs.

Since only TCP_* is reserved as part of tcp.h's namespace, we hide
them behind _BSD_SOURCE (and therefore _DEFAULT_SOURCE) to expose
them by default, but keep it standard conforming.

used by monitoring applications such as ss from iproute2
introduced in linux 3.15 commit 977cb0ecf82eb6d15562573c31edebf90db35163

policy is to avoid using these types except where they are needed for
namespace conformance. C99-style stdint.h types should be used
instead.

there are two versions of this structure: the BSD version and the GNU
version. previously only the GNU version was supported. the only way
to support both simultaneously is with an anonymous union, which was a
nonstandard extension prior to C11, so some effort is made to avoid
breakage with compilers which do not support anonymous unions.

this commit is based on a patch by Timo Teräs, but with some changes.
in particular, the GNU version of the structure is not exposed unless
_GNU_SOURCE is defined; this both avoids namespace pollution and
dependency on anonymous unions in the default feature profile.

the BSD and GNU versions of this structure differ, so exposing it in
the default _BSD_SOURCE profile is possibly problematic. both versions
could be simultaneously supported with anonymous unions if needed in
the future, but for now, just omitting it except under _GNU_SOURCE
should be safe.

removed in linux-v3.10 in commit 1a2c6181c4a1922021b4d7df373bba612c3e5f04

alternatively, we could define it in sys/socket.h since SO* is
reserved there, and tcp.h includes sys/socket.h in extensions mode.

note that SOL_TCP is simply wrong and it's only here for compatibility
with broken applications. the correct argument to pass for setting TCP
socket options is IPPROTO_TCP, which of course has the same value as
SOL_TCP but works everywhere.

the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.

installation docs have also been updated to reflect this change.

patch by Isaac Dunham. matched closely (maybe not exact) to glibc's
idea of what _BSD_SOURCE should make visible.