onewire_parser: Add support for temperature offsets

onewire_parser: Add support for temperature offsets

Some temperature sensors may have a significant systematic error
offset on their readings. This patch makes it possible to define an
offset value that is added to the raw value before the result is
stored in the rrd database.

From now on it is assumed that each onewire path never contains white
space. If there is a white space within the server path, it is
expected that each string that is separated with white space contains
an option that is used to control the readout of the values returned
or available from the server.

At the moment only one option is supported; temperature offset. The
option string must contain "offset=" following a decimal number. The
offset value is added to the number read from the server.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>