src/rrd_graph.[ch]: Implemented the ":daemon=..." option for "DEF:" commands.

src/rrd_graph.[ch]: Implemented the ":daemon=..." option for "DEF:" commands.

Rather than calling "FLUSH" for each RRD file needed for drawing a
graph and then accessing the file directly, use the "FLUSH" command to
receive the data required for drawing a graph.

The daemon can now be specified in one of three ways:

  * A new option has been added to the "DEF" option. If the "DEF" command
    includes the option "daemon=...", then the given daemon is used for
    accessing that file only. This option takes precedence over all
    other options. The value given follows the same syntax as other ways
    to specify a daemon address, thouh colons need to be escaped from
    rrdgraph(1).

    Example:
      'DEF:v1=path/to/example.rrd:value:AVERAGE:daemon=collect1.octo.cx'

  * The "--daemon" option can be given to the overall "rrdtool graph"
    command. When given, this address is used for all file accesses
    except those which explicitely contain a "daemon=..." option.

  * If the "RRDCACHED_ADDRESS" environment variable is set, it's value
    is used as the daemon address. This is the lowest priority option
    and only used if no other option is given.

One nice detail is that with ":daemon=...", the cache address can be
specified for each "DEF:" definition. It is therefore possible to graph
values stored on several servers in one graph – even if several RRD
files have the same name.