un-break undo_ifconfig_ipv4()/_ipv6() on all non-linux/non-win32 platforms

un-break undo_ifconfig_ipv4()/_ipv6() on all non-linux/non-win32 platforms

This commit needs a somewhat longer background story to explain the
problem...

undo_ifconfig_ipv4()/_ipv6() started their life as part of the
TARGET_LINUX (only) close_tun() function.

In commit 611fcbc48, these functions were created, to decouple IPv4/IPv6
dependency, still TARGET_LINUX only, with an #ifdef ENABLE_IPROUTE
inside, to differenciate iproute2 vs. old-style ifconfig.

Commit dc7fcd714 changed this to "the new linux API" (sitnl), calling
net_addr_ptp_v4_del() etc. - in the first branch of the #ifdef,
changing from ENABLE_IPROUTE to TARGET_LINUX, inside a TARGET_LINUX,
so the #else branch was never looked at for any platform.  The code
in that #else branch was still "the old linux ifconfig" style to
undo IPv4/IPv6 address config on the tun interface.

Now, commit 0c4d40cb8 comes along and makes undo_ifconfig_ipvX() a
global function, during the bugfix to "don't undo ifconfig if
--ifconfig-noexec is in effect".  Due to "it makes the code a lot
cleaner" undo_ifconfig*() is now called from do_close_tun_simple()
and no longer from (Linux-) close_tun().

*This* now enables the old "linux ifconfig" code to be run on
"all non-windows platforms" - running commands like

   ifconfig tun0 0.0.0.0

to remove the IPv4 address - which plain doesn't work on the BSDs
(and has not been tested anywhere else).

This all said, it's debatable whether any platforms actually NEED
this - all unixoid platforms remove IPv4/IPv6 addresses on interface
destroy time, so for non-persistant tun/tap interfaces, there is no
hard requirement to remove IP addresses on program exit.  For
persistent tun/tap (pre-create with "ifconfig tun7 create") this is
indeed useful to restore the pre-openvpn state by removing anything
OpenVPN configured.

OpenVPN up to 2.5 did not do this IP address removal on any non-Linux
platform, which is better than exec'ing an ifconfig command that does
nothing but print an error message (very annoying in t_client.sh V=1 runs).

This all said: this patch brings an implementation of undo_ifconfig_*()
for TARGET_FREEBSD ("ifconfig tunX $ip -alias"), and brings back the
old "do nothing" behaviour for all other unixoid platforms.  Tested
on FreeBSD 7.4, 12.3, 14.0.

v2: use #elif defined(TARGET_FREEBSD), otherwise it breaks other platforms

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20221004153127.527-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25337.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>