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/FullDevice.cpp

@@ -50,7 +50,7 @@ KResultOr<size_t> FullDevice::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;
 }
 
@@ -58,7 +58,7 @@ KResultOr<size_t> FullDevice::write(FileDescription&, size_t, const UserOrKernel
 {
     if (size == 0)
         return 0;
-    return KResult(-ENOSPC);
+    return ENOSPC;
 }
 
 }