Adding strsplitsep() to src/common.c

The new `strsplitsep()` function tokenizes a string into parts and
stores pointers to the parts in `fields`.

This is a generalization of the already-existing `strsplit()`. Instead
of wiring the delimiter string, it receives the `sep` argument (a string
with the separators) used to tokenize the given string.

To minimize code, the `strsplit()` function now becomes a wrapper to the
more general `strsplitsep()` with a fixed `sep` of `" \t\r\n"`.

NOTE: This patch will be used by subsequent changes in the IPMI plugin.
ipmi plugin: Adding support to override the default sensor naming on a per-sensor basis

The IPMI plugin sets a default naming policy for sensors. This feature
allows the user to override the default naming on a per-sensor basis.

To override the default sensor name simply add the custom name after the
default name in the config, as follows:

    Sensor "default_sensor_name;custom_sensor_name"

Note that the `default_sensor_name` is the default name given by the IPMI
plugin. At the moment, there is no easy way to know it beforehand, other
than by running the IPMI plugin and checking the names it assigns to the
sensors.

In the future, a helper tool should exist to list all of the available
sensors with their default IPMI `default_sensor_name`.
ipmi plugin: Adding 'IgnoreSensorFail' option to avoid removing sensors that fail

Before, the IPMI plugin would remove a sensor when a single read failure
occurred. With this option set to `true`, the sensor won't be removed
after failing (i.e., the IPMI plugin will retry reading from the sensor
the next time it gathers data).

By default, it's set to `false`, leaving the default behavior as it was
before the change (i.e. removing sensors even if they fail one reading).