Add support for data logging

Add support for data logging

Add new configuration parameters "logfile" and
"logfile_timestamp_fmt". Both of them are fed through strftime. The
first one is used for creating log file name, the second one is the
time stamp format in the beginning of each line.

Neither of the options are mandatory. If no file name is given, no
logging will be done. If no time stamp format is given, default of
"%Y.%m.%d %H:%M " is used.

Each line in the log file contains all the data elements given to
rrdtool during updating the database. The format is altered
slightly. The beginning of the line has got the time stamp, the
beginning "N:" part is removed and all ':' characters are replaced
with white space. Each entry is padded up to 12 characters with white
space in order to make the log file entries nicely human readable.

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