Use neubot-support/init scripts whenever possible

This commit uses the prepare.sh, start.sh, and stop.sh scripts
from mlab-support/init, which were lightly modified versions of
the namesake scripts in the current directory.

The prepare.sh script in mlab-support/init seems, instead, to
be quite different from my script, therefore for now I will
keep using the prepare.sh script version that I wrote for myself.