Enhanced Linux memory monitoring

The bundled default check is not very good for Linux. I think the one that comes with Nagios , check_snmp_mem.pl, is better. It takes cache memory and swap into consideration.

Add check command defintion for check_snmp_mem.pl:

Sample test output:

The changes are highlighted in red:

Modify the graph options since the perfdata format is different from the Centreon check plugin.

See Setting up custom graph.