Kernel+LibC: Turn errno codes into a strongly typed enum

..and allow implicit creation of KResult and KResultOr from ErrnoCode.
This means that kernel functions that return those types can finally
do "return EINVAL;" and it will just work.

There's a handful of functions that still deal with signed integers
that should be converted to return KResults.

Kernel/Devices/ZeroDevice.cpp

@@ -48,7 +48,7 @@ KResultOr<size_t> ZeroDevice::read(FileDescription&, size_t, UserOrKernelBuffer&
 {
     ssize_t count = min(static_cast<size_t>(PAGE_SIZE), size);
     if (!buffer.memset(0, count))
-        return KResult(-EFAULT);
+        return EFAULT;
     return count;
 }