move write_rrd and write_fh into rrd_create.c.
While this is purely cosmetic, writing an RRD file is actually creating one, so
it makes sense to concentrate the various writing functions into one place.
fix init_cdp. The original code implicitly always used the first pdp_prep
structure of the underlying rrd, which lead to wrong cdp initialization in
experimental code. This change will be needed for future work.
add rrd_init_data function
Extend rrd_create_r2 with a new (as of now unused) sources argument
- add support for --source options(s)
- add "destruction" path in rrd_create command line parser
turn GList based sources list into C array
add new rrdc_create_r2 function to support new sources syntax
add tests for no-overwrite functionality
Get rid of rrd_create_fn and use new creation infrastructure instead.
Keep a pointer to the internal rrd data structure with rrd_file_t objects
to simplify management of opened RRD files - now it is only required
to keep the rrd_file_t object around to be able to manage the underlying
rrd structure and not both objects in some additional data structure.
Add a dump-restore test
Move custom logic row-rotation logic from write_fh, originally there to
support rrd_restore with its random cur_row placement to
rrd_restore itself.  This makes write_fh a truly reuseable function.
allow to print the actual rrdtool calls run by a test by setting the VERBOSE
env. variable to a non-empty string before calling the test
Move some functions from rrd_modify to rrd_create
some minor fixes for correctness
first real prefill implementation
- this still contains loads of debug output and some real problems and
  assumptions, so it won't work correctly for most cases
First test for create with old data
reduce number of warnings - even if most are triggered by debug code
properly handle DS names - this allows to reorder data sources
Add interval handling functions. These will be needed in the future to
handle pre filling of RRAs.