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/FileSystem/InodeWatcher.cpp

@@ -77,14 +77,14 @@ KResultOr<size_t> InodeWatcher::read(FileDescription&, size_t, UserOrKernelBuffe
         return (ssize_t)data_bytes;
     });
     if (nwritten < 0)
-        return KResult(nwritten);
+        return KResult((ErrnoCode)-nwritten);
     evaluate_block_conditions();
     return bytes_to_write;
 }
 
 KResultOr<size_t> InodeWatcher::write(FileDescription&, size_t, const UserOrKernelBuffer&, size_t)
 {
-    return KResult(-EIO);
+    return EIO;
 }
 
 String InodeWatcher::absolute_path(const FileDescription&) const