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.