[kernel] Re-add channel syscalls

Channels were unused, and while they were listed in syscalls.def, they
had no syscalls listed in their interface. This change adds them back,
and updates them to the curren syscall style.

src/kernel/syscalls/channel.cpp

@@ -1,5 +1,6 @@
 #include <j6/errors.h>
 #include <j6/types.h>
+#include <util/counted.h>
 
 #include "objects/channel.h"
 #include "syscalls/helpers.h"
@@ -16,15 +17,37 @@ channel_create(j6_handle_t *self)
 }
 
 j6_status_t
-channel_send(channel *self, size_t *len, void *data)
+channel_send(channel *self, void *data, size_t *data_len)
 {
-    return self->enqueue(len, data);
+    if (self->closed())
+        return j6_status_closed;
+
+    const util::buffer buffer {data, *data_len};
+    *data_len = self->enqueue(buffer);
+
+    return j6_status_ok;
 }
 
 j6_status_t
-channel_receive(channel *self, size_t *len, void *data)
+channel_receive(channel *self, void *data, size_t *data_len)
 {
-    return self->dequeue(len, data);
+    if (self->closed())
+        return j6_status_closed;
+
+    util::buffer buffer {data, *data_len};
+    *data_len = self->dequeue(buffer);
+
+    return j6_status_ok;
+}
+
+j6_status_t
+channel_close(channel *self)
+{
+    if (self->closed())
+        return j6_status_closed;
+
+    self->close();
+    return j6_status_ok;
 }
 
 } // namespace syscalls