tapctl: Resolve MSVC C4996 warnings

tapctl: Resolve MSVC C4996 warnings

wcsncat() was declared unsafe in favour of wcsncat_s(). However, the
string concatenation follows the string length check, making wcsncat()
safe too. Code analysis is just not smart enough (yet) to detect this.

The code was refactored to use wcscat_s() MSVC is considering as "safe".

Signed-off-by: Simon Rozman <simon@rozman.si>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210322074359.527-1-simon@rozman.si>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21774.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e5e9a07e8baee4065b7dfd65736bfa77b8329cfc)