Cygwin: Fix return value of sched_getaffinity

Have sched_getaffinity() interface like glibc's, and provide an
undocumented internal interface __sched_getaffinity_sys() like the Linux
kernel's sched_getaffinity() for benefit of taskset(1).

winsup/cygwin/include/sys/cpuset.h

@@ -9,6 +9,10 @@ details. */
 #ifndef _SYS_CPUSET_H_
 #define _SYS_CPUSET_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef __SIZE_TYPE__ __cpu_mask;
 #define __CPU_SETSIZE 1024  // maximum number of logical processors tracked
 #define __NCPUBITS (8 * sizeof (__cpu_mask))  // max size of processor group
@@ -22,4 +26,10 @@ typedef struct
   __cpu_mask __bits[__CPU_GROUPMAX];
 } cpu_set_t;
 
+int __sched_getaffinity_sys (pid_t, size_t, cpu_set_t *);
+
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _SYS_CPUSET_H_ */