Add github actions

Add github actions

dummy0 gives strange errors on the Ubuntu 16 runner on github actions
because
dummy already exist, so use a more unique ovpn-dummy0 name instead.

Github actions are a good alternative to travis-ci, which futrure is
questionable
at the moment without payment. The github actions also allows building on
macOS
and Windows (not included in this commit). The  matrix is a bit different
than Coverity and uses different Ubuntu version with their native OpenSSL
(1.0.2, 1.1.1)/mbed TLS instead of manually compiling different OpenSSL
versions on just Ubuntu 20.04.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20210608152434.1403999-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22511.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>