security/openvpn: reliability fixes cherry-picked from upstream

Arne Schwabe's OpenSSL fix for Debian Bug#958296
"Fix tls_ctx_client/server_new leaving error on OpenSSL error stack"
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=958296 [1]

Selva Nair's auth-pam fixes
"Parse static challenge response in auth-pam plugin"
"Accept empty password and/or response in auth-pam plugin"

Re-diff (with make makepatch) older patches.

Reported by: Jonas Andradas via Debian BTS
Obtained from: Arne Schwabe, Selva Nair https://github.com/OpenVPN/openvpn/tree/release/2.4
MFH: 2020Q2 (blanket for backporting reliability fixes)