send uname() release as IV_PLAT_VER= on non-windows versions

send uname() release as IV_PLAT_VER= on non-windows versions

This is highly system specific, as the content of the uname()
structure elements is not specified very well - uname(3) says:

      release       Release level of the operating system

which translates to "IV_PLAT_VER=13.3-RELEASE-p6" (FreeBSD) or
"IV_PLAT_VER=22.6.0" (macOS) - the latter being the "Mach Kernel
version", not what Apple calls the OS.

It's still useful if a server operator needs to keep track of
client versions (and the GUI does not set the corresponding
environment variable, which neither Tunnelblick nor NM do).

v2: manpage amendments
v3: whitespace
v4: reword manpage

Github: OpenVPN/openvpn#637

Change-Id: Id2b0c5a517f02e5c219fea2ae3ef2bdef7690169
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20241104085808.17039-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29699.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit d5b4713153ce95e88651d93aa1ef25b2676d0cfc)