Simplify --genkey option syntax

Simplify --genkey option syntax

Instead of requiring users to do "--genkey --secret new.key", allow
them to just do "--genkey new.key".  This has hit me often enough that I
decided to write a patch for it.  Also, the upcoming tls-crypt-v2-genkey
uses a similar syntax and Antonio suggested we should make them consistent.

The documentation is updated to no longer mention the old syntax, but it is
still supported so people who are used to the old syntax can still use it.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20181005150032.16541-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17574.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>