Include an OpenIKED Vendor ID payload in the initial handshake. This will

Include an OpenIKED Vendor ID payload in the initial handshake.  This will
make it easier to handle interoperability problems with older versions in
the future.  The ID is constructed from the string "OpenIKED-" followed by
the version number.
Sending of the vendor ID payload can be disabled by specifying
"set novendorid" in iked.conf(5).

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