Implement getloadavg()

v2:
autoload PerfDataHelper functions
Keep loadavg in shared memory
Guard loadavg access by a mutex
Initialize loadavg to the current load

v3:
Shared memory version bump isn't needed if we are only extending it
Remove unused autoload
Mark inititalized flags as NO_COPY for correct behaviour in fork child

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>

winsup/cygwin/include/cygwin/stdlib.h

@@ -77,6 +77,10 @@ extern _PTR valloc _PARAMS ((size_t));
 #undef _mstats_r
 #define _mstats_r(r, p) mstats (p)
 
+#if __BSD_VISIBLE
+int getloadavg(double loadavg[], int nelem);
+#endif
+
 #ifdef __cplusplus
 }
 #endif