`collectd' might fail because of a failure in any of numerous plugins.
For example `virt' plugin sometimes fails if `collectd' is started before `libvirtd'
* removed pid-file support, it is needless to run collectd as systemd service
* removed static user id, as all the files reowned on the service start
* added ambient capabilities for ping and smart (hdd health) functions
The collectd service runs as an unprivileged user by default, so it does
not leak more information to its data directory than any user can obtain
elsewhere by other means.
If people are running it as root and are worried about information leak,
we can add collectd group and set perms to 750.
CC @offlinehacker.
Fixes#21198.
