The output file of generate_classvar_mapping (classvar_mapping) used
to be closed after the temporary file had been renamed. This seems to
have caused the output file to be empty on some platforms. The file is
now closed before it is renamed.

This update will reduce the classvar mapping lists generated from 
virtual mapping files.

Previously all destination values where always extracted. Now the
EXACT_MAPPINGS section is first scanned to find all possible destination
values for every source value and then when the data is scanned only the
potential destination values are added to the classvar mapping output.

This improvement will usually give significant reductions in the size of 
the classvar_mapping file when the mapping files are automatically created
from external sources.

In large installations and installations that have been running for
some time, the amount of classification values will be so large that
it might cause the Reporter Professional to experience memory
problems.

This patch makes it possible to restrict the amount of data that is
inspected to generate these value lists. More specifically it allows
you to specify how far back (in years) the generator should go to
extract classification values from the data in the database.

It introduces the configuration directive CLASSVAR_CUTOFF_AGE, to be
used in /etc/opt/openit/openit.cfg. The value specified will be the
number of years to inspect. 1 = the current year only. 2 = current
year and last year, and so forth. 0 = inspect all data available.

The default value, if none is specified, is 2, i.e. to inspect data
from the current year and last year.