fix return value of nice function

the Linux SYS_nice syscall is unusable because it does not return the
newly set priority. always use SYS_setpriority. also avoid overflows
in addition of inc by handling large inc values directly without
examining the old nice value.

src/unistd/nice.c

 #include <unistd.h>
 #include <sys/resource.h>
+#include <limits.h>
 #include "syscall.h"
 
 int nice(int inc)
 {
-#ifdef SYS_nice
-	return syscall(SYS_nice, inc);
-#else
-	return setpriority(PRIO_PROCESS, 0, getpriority(PRIO_PROCESS, 0)+inc);
-#endif
+	int prio = inc;
+	// Only query old priority if it can affect the result.
+	// This also avoids issues with integer overflow.
+	if (inc > -2*NZERO && inc < 2*NZERO)
+		prio += getpriority(PRIO_PROCESS, 0);
+	if (prio > NZERO-1) prio = NZERO-1;
+	if (prio < -NZERO) prio = -NZERO;
+	return setpriority(PRIO_PROCESS, 0, prio) ? -1 : prio;
 }