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

winsup/cygwin/include/cygwin/version.h

@@ -473,12 +473,13 @@ details. */
   306: Export getentropy, getrandom.
   307: Export timingsafe_bcmp, timingsafe_memcmp.
   308: Export dladdr.
+  309: Export getloadavg.
 
   Note that we forgot to bump the api for ualarm, strtoll, strtoull,
   sigaltstack, sethostname. */
 
 #define CYGWIN_VERSION_API_MAJOR 0
-#define CYGWIN_VERSION_API_MINOR 308
+#define CYGWIN_VERSION_API_MINOR 309
 
 /* There is also a compatibity version number associated with the shared memory
    regions.  It is incremented when incompatible changes are made to the shared