relibc: implement clock_settime() via /scheme/time

Replaced ENOSYS stub with real implementation: opens
/scheme/time/{clk_id} for writing and writes the timespec.
Cross-referenced with Linux 7.1 kernel/time/posix-clock.c
pc_clock_settime().

The kernel's /scheme/time scheme supports write() for both
CLOCK_REALTIME and CLOCK_MONOTONIC. Writing a TimeSpec sets
the clock value. Returns EIO if fewer bytes written than
expected.

src/platform/redox/mod.rs

@@ -264,8 +264,31 @@ impl Pal for Sys {
     }
 
     unsafe fn clock_settime(clk_id: clockid_t, tp: *const timespec) -> Result<()> {
-        todo_skip!(0, "clock_settime({}, {:p}): not implemented", clk_id, tp);
-        Err(Errno(ENOSYS))
+        // Write the timespec to /scheme/time/{clk_id} to set the clock.
+        // Cross-referenced with Linux 7.1 kernel/time/posix-clock.c
+        // pc_clock_settime() which writes to the clock device.
+        if tp.is_null() {
+            return Err(Errno(EFAULT));
+        }
+        let ts = unsafe { *tp };
+        let path = format!("/scheme/time/{}", clk_id as usize);
+        let fd = redox_rt::sys::open(
+            &path,
+            syscall::O_WRONLY | syscall::O_CLOEXEC,
+            0,
+        ).map_err(|e| Errno(e.errno))?;
+        let bytes = redox_rt::sys::write(fd, unsafe {
+            core::slice::from_raw_parts(
+                &ts as *const timespec as *const u8,
+                size_of::<timespec>(),
+            )
+        }).map_err(|e| Errno(e.errno))?;
+        let _ = redox_rt::sys::close(fd);
+        if bytes == size_of::<timespec>() {
+            Ok(())
+        } else {
+            Err(Errno(EIO))
+        }
     }
 
     fn close(fd: c_int) -> Result<()> {