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/TTY/MasterPTY.cpp

@@ -75,7 +75,7 @@ KResultOr<size_t> MasterPTY::read(FileDescription&, size_t, UserOrKernelBuffer&
 KResultOr<size_t> MasterPTY::write(FileDescription&, size_t, const UserOrKernelBuffer& buffer, size_t size)
 {
     if (!m_slave)
-        return KResult(-EIO);
+        return EIO;
     m_slave->on_master_write(buffer, size);
     return size;
 }