Fix a couple of issues in openvpn_execve()

Fix a couple of issues in openvpn_execve()

If openvpn_execve() is not able to fork(), it would not make any noise
about it.  So this patch adds a log notification if this happens.

In addition, if openvpn_execve() is called with an empty argv array,
it should exit instantly.  This is not expected to happen at all and
might indicate a much more serious issue (or programming error)
somewhere else in the code.  Thus, abort execution to get these issues
flushed out as quickly as possible.

Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Gert Doering <gert@greenie.muc.de>