[CE 14.4/SVN 5185] Disable threshold monitoring on port types

Suggested improvement to poller PHP logic and additional configuration file option [alerts][port_util_ignore_ifdescr].

Prevents AToM/SAToP/Circuit Emulated transport over MPLS (Cisco seem to denote these ports as CEMn/n/n where n is the module/port/interface numnber) from constantly putting threshold alerts into the eventlog (because Circuit Emulation always seems to run at 100% of the emulated circuit type, i.e. E1/T1 is always 2 Mbps etc).

New config file option:
$config['alerts']['port_util_ignore_ifdescr'] = array("CEM"); // Interface types to ignore threshold alerting for (i.e. CEM = T1/E1 Circuit Emulation, AToM)