Use setpriv instead of gosu to drop privileges

Changes:

setpriv is used instead of gosu with the following flags:

* Set reuid and regid to redis user and group
* Clear all supplementary groups
* Set bounding capabilities to an empty list
* Enable no-new-privs bit

redis-sentinel is now also run with dropped privileges (previously, it wasn't)

Both redis-sentinel and redis-server will start with dropped privileges, regardless of how they were started (whether using absolute paths or just file names)

Author: Peter-Sh