Use network address for emulated DHCP server as a default

Use network address for emulated DHCP server as a default

This is the rebase of original Selva Nair's patch
which hasn't been merged:

  https://sourceforge.net/p/openvpn/mailman/message/34674818/

and documentation change to reflect code changes, which
is basically a revert of another Selva's patch (which got merged):

https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13387.h
tml

For subnet topology use "offset 0" as default for
calculating DHCP server address, which makes it equal
to the network address.

There is no know reason why non-zero default offset
is needed. Besides, offset -1 breaks subnet /30 case,
which in some cases is pushed by OpenVPN Cloud product.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211109015927.311-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23156.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>