sockets: remove unused KPIs to manipulate sockets

These KPIs were added in dd0e6c383a9f0 and through 15 years had zero use.
They slightly remind what IfAPI does for struct ifnet. But IfAPI does
that for the sake of large collection of NIC drivers not being aware of
struct ifnet. For the sockets it is unclear what could be a large
collection of externally written kernel modules that need extensively use
sockets and not be aware of their internals at the same time. This
isolation of a structure knowledge requires a lot of work, and just
throwing in a few KPIs isn't helpful.

Reviewed by: kib, olce, markj
Differential Revision: https://reviews.freebsd.org/D44311

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Use stub inline functions for no-op versions of tcp_fastopen*().

Inline functions "use" variables passed as arguments unlike empty
macros appeasing compiler warnings about unused variables.

Greatly reduce the number of #ifdefs supporting the TCP_RFC7413 kernel option.

The conditional compilation support is now centralized in
tcp_fastopen.h and tcp_var.h. This doesn't provide the minimum
theoretical code/data footprint when TCP_RFC7413 is disabled, but
nearly all the TFO code should wind up being removed by the optimizer,
the additional footprint in the syncache entries is a single pointer,
and the additional overhead in the tcpcb is at the end of the
structure.

This enables the TCP_RFC7413 kernel option by default in amd64 and
arm64 GENERIC.

Reviewed by: hiren
MFC after: 1 month
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14048

This is an implementation of the client side of TCP Fast Open (TFO)
[RFC7413]. It also includes a pre-shared key mode of operation in
which the server requires the client to be in possession of a shared
secret in order to successfully open TFO connections with that server.

The names of some existing fastopen sysctls have changed (e.g.,
net.inet.tcp.fastopen.enabled -> net.inet.tcp.fastopen.server_enable).

Reviewed by: tuexen
MFC after: 1 month
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14047

Implementation of server-side TCP Fast Open (TFO) [RFC7413].

TFO is disabled by default in the kernel build. See the top comment
in sys/netinet/tcp_fastopen.c for implementation particulars.

Reviewed by: gnn, jch, stas
MFC after: 3 days
Sponsored by: Verisign, Inc.
Differential Revision: https://reviews.freebsd.org/D4350