Dear Björn,
The supported ssl driver for NaviServer is nsssl [1] part of the 43 NaviServer modules [2]. Nsssl is up to date in terms of the security features of OpenSSL (e.g. forward secrecy). When using this driver, one gets an A+ rating from SSL labs by following the configuration instructions in the README file. Furthermore, the driver nsssl provides the "ns_ssl" command for https client requests, compatible with "ns_http", which use the same c-level infrastructure.
The compilation problem you faced with nsopenssl are due to the fact, that this driver was not updated for Tcl 8.6. Your fix is partly correct, but setting the interp result has to go through the appropriate tcl-api to set the result (e.g. Tcl_SetResult). I am not eager for fixing this, since we don't use the driver at all.
Actually, we are using on all production environments still Tcl 8.5, also OpenACS recommends still Tcl 8.5. There are no known issues with Tcl 8.6, just a thorough testing is missing.
all the best
-gustaf neumann
[1] https://bitbucket.org/naviserver/nsssl
[2] http://sourceforge.net/projects/naviserver/files/naviserver/4.99.6/naviserver-4.99.6-modules.tar.gz/download