Fix rrdc_fetch behavior on RRDs with multiple DSes

When reading values from RRD files with multiple data sets, the auxiliary
parsing function would only copy the first item from the returned list; the
others were left uninitialized. On my x64_64 Linux machine, this actually reused
values form previous call to parse_char_array_header which happens to be a DS
definition. Given a RRD file with DS "a", "b" and "c", the first value on each
line would be correctly parsed, but the second value is "b", the actual DS name,
and the third one "c". These "values" cannot be parsed as numbers, and the whole
function therefore returns with an error. Making sure that all pointers within
the array are copied therefore fixes this problem.
Merge pull request #445 from jktjkt/fix-rrdc-fetch

Fix rrdc_fetch behavior on RRDs with multiple DSes
rrd_graph: initialize new data member (fix #441)

primary_axis_format was added to image_desc_t but rrd_graph_init was not
updated to initialize it, resulting in this error when the garbage
values were used as axis labels:

(process:24893): Pango-WARNING **: Invalid UTF-8 string passed to pango_layout_set_text()
autogen.sh: use POSIX comparison for test

Although bash recognizes ==, and the shell has a shebang to use it, some
of us still automatically type "sh autogen.sh" instead of
"./autogen.sh", and this breaks on debian/jessie where /bin/sh is not
bash.  The fix is trivial and should be safe for bash too (which
recommends using = instead of == with the test command for POSIX
conformance.
make write_file public
add rrd_modify (but do not yet use it)
make modify command available
- so far it is only really possible to remove data sources
Refactor all DS definition parsing code. It was not reusable and tied
too much to the rrd_t data structure.
Allow to add data sources to an RRD
Extend rrdcached client API to be able to also send FORGET and other
filename based commands easily
provide new write_fh function
streamline and clean up
Add some documentation
rrdcached handling
Add documentation for modify command
Merge pull request #447 from stamfest/rrdmodify-master

Rrdmodify master
Add support for rrd restore to explicitly end xml files when using
the pipe interface by using the "-" special filename and using
ctrl-Z/newline as an end-of-file indicator.
Fix double-free bug
Avoid endless looping in case of XML errors.
suppress error message from libxml when reading XML from stdin.
This avoids breakage of the pipe protocol in case of errors.