t_net.sh: drop hard dependency on t_client.rc

t_net.sh: drop hard dependency on t_client.rc

Right now t_net.sh depends on t_client.rc in order to source the
RUN_SUDO variable only.
However, t_client.rc is something that a few people only have configured
and thus this would result in t_net.sh almost never executed even if it
just could.

Drop dependency on t_client.rc by falling back to RUN_SUDO=sudo when the
file is missing and no RUN_SUDO is passed via env.

While at it, reword the error message to better match the current logic
flow.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200721195518.14358-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20533.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>