Change '--multihome' behaviour regarding egress interface selection.

Change '--multihome' behaviour regarding egress interface selection.

Traditional OpenVPN ``--multihome`` behaviour is to send packets out the
same interface that they were received on (copy ipi_ifindex from ingress
to egress packet info).  For some scenarios this makes sense, for other
scenarios it is breaking connectivity when there are no routes pointing
out the ingress interface (intentionally asymmetric traffic).

For 2.7.0, change the default(!) to always send out packets with
ipi_ifindex = 0, to follow normal system interface selection rules.

Add a flag ``--multihome same-interface`` to restore the pre-2.7 behavior
of copying ipi_ifindex from ingress to egress packets.  There are use
cases for this, and we want to give users a chance to read the release
notes and adjust their setups to "not break after upgrading to 2.7.0".

Github: OpenVPN/openvpn#855
Github: OpenVPN/openvpn#554

v2: fix whitespace
v3: turn logic around - new default is "egress ifindex 0" now
v4: typo fixed in commit message
v5: fix invalid rst in Changes.rst

Change-Id: Id429241e1b17a8ff51d9019efc357c910f3bde4c
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1383
Message-Id: <20251126130410.19091-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34709.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>