implemented openbsd netspeed functions

implemented the netspeed functionality for openbsd.
furthermore the static keyword was removed of the interval variable in
config.def.h for usage as extern variable.

config.def.h

@@ -1,7 +1,7 @@
 /* See LICENSE file for copyright and license details. */
 
 /* interval between updates (in ms) */
-static const unsigned int interval = 1000;
+const unsigned int interval = 1000;
 
 /* text to show if no value can be retrieved */
 static const char unknown_str[] = "n/a";