Refactor of service for systemd 229

Since introduction of ambient capabilities,
systemd deprecated the use of Capabilities.
With systemd 229 activated with krogoth,
the use of Capabilities does nothing.

This commits avoids to use SecureBits and Capabilities.
It now relies on the fact that post installations are
setting the capabilities to the file:

 - setcap cap_mac_override,cap_dac_override=ep          afm-system-daemon
 - setcap cap_mac_override,cap_mac_admin,cap_setgid=ep  afm-user-daemon

Using p (permitted) instead of i (inherited) that was
previously used.

Change-Id: I17d51da07adf775d47a7f44551d8b8ebbc6eacb9
Signed-off-by: José Bollo <jose.bollo@iot.bzh>