Some environments, like system containers, have no reason to have
sudo(8) installed. In these environments restart.sh will silently fail
because /usr/bin/sudo does not exist to execute, so test that sudo
exists and don't try to use it otherwise.
Note also that hardcoding sudo's path is wrong: it can be installed in
other places. On FreeBSD, for example, it is /usr/local/bin/sudo when
installed from ports.
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