graph: allow RRD files to be missing

An option (-Z/--missing-ds) is added to allow RRD files to be
missing. In this case, the unknown value is assumed.
graph: when an RRD file is missing, provide an empty one

Instead of providing a RRD without any DS, we provide one with the
expected DS. This allows one to use CDEF with this RRD. The DS
contains only NaN values.
graph: rename --missing-ds to --use-nan-for-all-missing-data
Fixed constant used in timespan check to be a long long.

This fixes a compiler warning on AIX in 32-bit with IBM XL C/C++ compiler.
Add an option to let rrdcached recursively create directories.

If a `create` command is given, a path is accepted. This option enables rrdcached to create the directory structure if it does not exist yet.
Change tabs to spaces
Fix vertical legend alignment for --legend-direction=bottomup

Issue 419
Add a --legend-direction=bottomup variant

The --legend-direction=bottomup2 variant will exempt leading and
trailing COMMENT lines from re-ordering. This might be useful for
generators like munin that use such lines for table headers and the
like.

Issue 419
Update rrd_fetch.c
Merge pull request #423 from stevenh/master

Add rrdcached client support to windows + misc compatibility fixes
Merge pull request #424 from crossett/master

Allow sql// and sql|| (pipe pipe) for libdbi datasources
Merge pull request #422 from cbiedl/master

Fix vertical legend alignment, enhance --legend-direction
Merge pull request #406 from nhale25/master

Fixed constant used in timespan check to be a long long.
Restrict -R usage to when -B (and -b) are set.
Update rrdcached documentation with new switch and its usage.
Merge pull request #414 from Zsub/feature/rrdcached-create-recursive

Add an option to let rrdcached recursively create directories.
Merge pull request #384 from vincentbernat/feature/allow-inexistent-files

graph: allow RRD files to be missing
xport should only return as much data as was requested
initialize DEF step with chart --step when looking for data fix for a regression with the new argument parser
argument parser regression fix - if no explicit reduce function is set, use the one associated with the RRA and not AVERAGE