Introduce support for SNMPv3 context in check_snmp

snmpget already supports using SNMPv3 contexts using the option "-n".
Thus all we need to do is introduce a new argument to check_snmp and
pass the argument on to snmpget using snmpget's option "-n".
Since "-n" is already in use for check_snmp for a different purpose,
we use "-N" instead.
NEWS: Mention new check_snmp option

Closes #1221.