[runit] don't use chpst when starting runsvdir

If runsvdir is started as a non-root user, it won't be able to use
  chpst to change the effective uid when starting any subsequent
  services, even if the effective uid is the same as the one runsvdir
  is running as.
[runit] use $SENSU_USER instead of $USER in sv run script

Many other programs look at $USER env var to figure out who they are
running as. Let's avoid any weirdness and use $SENSU_USER instead.
[runit] store the original path before we add embedded/bin to it
[runit] modify sv run script logic to use $ORIGINAL_PATH exported by sensu-runsvdir
[runit] teaching package scripts to link sensu-ctl into /usr/bin