async callback confusion fixed #883

[stefan-baranoff]

For discussion on fixing #883 by back-porting f353b and other required code, which fixed #793 to the OpenVPN 2.6 line. This seems to fix the original issue see in but other issues were uncovered in the process that may apply to 2.6 and 2.7; but I wanted this code to be available in case it's useful.