hardening systemd security

[OniriCorpe]

Problem

• the systemd config file had no security options

Solution

• I have configured and tested the options given in example_ynh, I tweaked or removed some settings which prevented the proper functioning and everything works fine :)

PR Status

• [x] Code finished and ready to be reviewed/tested
• [x] The fix/enhancement were manually tested (if applicable)

[yunohost-bot]

:stuck_out_tongue_winking_eye: