Wrong Sys:Syslog usage

[Guillaume Rousse]

The Syslog backend in ocsinventory-agent is wrong in many ways:
1) it calls setlogsock(), whereas this is discouraged
2) it reopens syslog for every message, instead of keeping it open during the whole agent runtime
3) it uses USER environement variable as a facility name, instead of using standard syslog facility name
4) it doesn't allow to configure used facility

The attached patch fixes the various issues.

I fixed point 2 by using a destructor in Syslog backend to close the logs. An explicit method call could have been used too, as the problem is not specific to this backed. The file backend is also affected