Fixed issue where struct env_set methods that

Fixed issue where struct env_set methods that
change the value of an existing name=value pair
would delay the freeing of the memory held by
the previous name=value pair until the underlying
client instance object is closed.

This could cause a server that handles long-term
client connections, resulting in many periodic calls
to verify_callback, to needlessly grow the env_set
memory allocation until the underlying client instance
object is closed.

git-svn-id: http://svn.openvpn.net/projects/openvpn/branches/BETA21/openvpn@1493 e7ae566f-a301-0410-adde-c780ea21d3b5