Remove strerror_ts()

Remove strerror_ts()

This function was only called in string format functions, which already
copy the contents, so all this ever did was adding redundant malloc() and
free() calls.

Also, this wasn't as thread-safe as it claims: another thread could still
change the string value between the strerror() and buf_printf() calls. So,
instead of a not needed false sense of thread-safeness, just be honest and
use strerror() directly.

(I think we should find a better place for everything currently in misc.c,
and get rid of it all together.  In this case, the better place is
/dev/null.  This patch is part of that effort.)

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1500550740-24773-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15105.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit fd2a29ab2668fea9c0ac972d5ec69f00232c88b6)