sysctl: add --strict option to fail if sysctl does not exists

sysctl: add --strict option to fail if sysctl does not exists

systemd-sysctl currently fails silently under any of these conditions:
- Missing permission to write a sysctl.
- Invalid sysctl (path doesn't exists).
- Ignore failure flag ('-' in front of the sysctl name).

Because of this behaviour, configuration issues can go unnoticed as
there is no way to detect those unless going through the logs.

--strict option forces systemd-sysctl to fail if a sysctl is invalid or
if permission are insufficient. Errors on sysctl marked as "ignore
failure" will still be ignored.