dco: backport OS-independent part of peer float support

dco: backport OS-independent part of peer float support

This is a backport of commit cb8a0f6f5741d102b667d98370ab4d553503d0b5,
which introduces float support for DCO linux, Windows, and the
OS-independent parts.

DCO linux/windows in 2.6 has no float support kernel-side, so this
ignores all OS dependent parts, backporting just enough to add
FreeBSD support in the next patch.

One notable difference in the backport is that 2.6 has no multi-socket
support, so all the "link_sockets[0]" occurances need to be changed back
to "link_socket".

See https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=289303 for all
the epic details...

Change-Id: Ib748e726eb84dcbe8a48b297d165dec80c0e578d
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Ralf Lici <ralf@mandelbit.com>
(cherry picked from commit cb8a0f6f5741d102b667d98370ab4d553503d0b5)
Message-Id: <20250908081124.17933-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59230454/
Signed-off-by: Gert Doering <gert@greenie.muc.de>