Fixed a client-side bug that occurred when the "dhcp-pre-release"

Fixed a client-side bug that occurred when the "dhcp-pre-release"
or "dhcp-renew" options were combined with "route-gateway dhcp".

The problem is that the IP Helper functions for DHCP release and
renew are blocking, and so calling them from a single-threaded
client stops tunnel traffic forwarding, and hence breaks
"route-gateway dhcp" which requires an active tunnel.  The fix is
to call the IP Helper functions for DHCP release and renew from
another process.

Version 2.1_rc21b.

git-svn-id: http://svn.openvpn.net/projects/openvpn/branches/BETA21/openvpn@5164 e7ae566f-a301-0410-adde-c780ea21d3b5