snmp plugin: Parse strings according to the data source type.

snmp plugin: Parse strings according to the data source type.

Some broken SNMP implementations returns numbers as a string, e. g. a
temperature could would be returned as "19.2 C". This change uses the
data source type, i. e. `counter' or `gauge' to convert that string into
an integer or a floating point number using strtoll or strtod. In case
of an error (string-pointer is NULL or no conversion could take place)
either zero or NAN is returned.