configure: enable DCO by default on FreeBSD/Linux

configure: enable DCO by default on FreeBSD/Linux

Automatically disabled when
- iproute2 is enabled
  (Don't want to force people specifying --disable-dco explicitely)
- libnv is missing on FreeBSD
  (FreeBSD version too old anyway)

Will still error out if libnl-genl is missing on Linux to
make people aware of new dependency.

v2: error out when libnl-genl is missing as discussed with ordex on
    IRC.
v3:
 - improvements to the messages, suggested by Selva
 - further improvements to the default specification, trying to make it clear
 - if enabling iproute2, do not test for libnl-genl
v4: add updates for GHA
v5:
 - v4 was missing the changes of v3. v5 combines the changes from v3 and v4
 - fix build failure GHA/ubuntu1804/mbedtls
 - fix build failure GHA/ubuntu2204/libressl

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20230215162654.52137-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26272.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>