process: sys_wait4: return ECHILD with no children

If the calling process has no children and no pending exit events to
handle, return ECHILD instead of waiting forever.

libkernel/src/error/syscall_error.rs

@@ -53,6 +53,7 @@ pub fn kern_err_to_syscall(err: KernelError) -> isize {
         KernelError::NotSupported => ENOSYS,
         KernelError::NoMemory => ENOMEM,
         KernelError::TimedOut => ETIMEDOUT,
+        KernelError::NoChildProcess => ECHILD,
         e => todo!("{e}"),
     }
 }