move dring to ${prefix}/sbin

${prefix}/libexec is not part of the FHS. Moving to ${prefix}/sbin to
accomodate distros

See https://lists.debian.org/debian-devel/2005/05/msg00401.html

Change-Id: I9b2911929a2f8cc2c8f624c56b6491883c71b6a8
Tuleap: #350