Thread safety: Replace getopt by optparse

Thread safety: Replace getopt by optparse

* Added optparse.h to src/rrd_tool.h.

* rrd_graph.[ch]: The 3rd char* parameter of the "*_conv" functions was added a const modifier.

* rrd_tune.c: The 3rd char* parameter of the "set_*" functions was added a const modifier.

* rrd_graph_options now takes an extra "options*" parameter, so that caller can use options.argc and options.argv

* Refresh WIN32 build tips