redbear-power: add set_nice() syscall helper for nice/renice editor

local/recipes/system/redbear-power/source/src/process.rs

@@ -1987,3 +1987,23 @@ mod io_sort_unit_tests {
         assert_eq!(read_cpu_affinity(999_999_999), None);
     }
 }
+
+
+/// Set the nice value of a process via setpriority(2). htop F7/F8 feature.
+pub fn set_nice(pid: u32, nice: i32) -> Result<(), String> {
+    let n = nice.clamp(-20, 19);
+    #[cfg(target_os = "linux")]
+    unsafe {
+        // PRIO_PROCESS = 0, who = pid
+        if libc::setpriority(0, pid as u32, n) == 0 {
+            Ok(())
+        } else {
+            Err(std::io::Error::last_os_error().to_string())
+        }
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        let _ = (pid, n);
+        Err("setpriority not supported on this platform".into())
+    }
+}