Tools: exit with code 0 when showing --help output

This is in line with what most tools do when printing --help
(and GOptionContext alike)

tools/gtk-builder-tool.c

@@ -46,7 +46,7 @@ usage (void)
              "  render       Take a screenshot of the file\n"
              "  screenshot   Take a screenshot of the file\n"
              "\n"));
-  exit (1);
+  exit (0);
 }
 
 static GLogWriterOutput

tools/gtk-image-tool.c

@@ -45,7 +45,7 @@ usage (void)
              "  relabel      Change the color state of the image without conversion\n"
              "  show         Show the image\n"
              "\n"));
-  exit (1);
+  exit (0);
 }
 
 static GLogWriterOutput

tools/gtk-path-tool.c

@@ -45,7 +45,7 @@ usage (void)
              "  render       Render the path as an image\n"
              "  info         Print information about the path\n"
              "\n"));
-  exit (1);
+  exit (0);
 }
 
 static GLogWriterOutput

tools/gtk-rendernode-tool.c

@@ -45,7 +45,7 @@ usage (void)
              "  show         Show the node\n"
              "  render       Take a screenshot of the node\n"
              "\n"));
-  exit (1);
+  exit (0);
 }
 
 static GLogWriterOutput