Changing the message union to anonymous.

Changing the message union to anonymous.

This allows us to write things like this:
  message m;
  m.m_notify.interrupts = new_value;

or
  message *mp;
  mp->m_notify.interrupts = new_value;

The shorthands macro have been adapted for the new scheme, and will be
kept as long as we have generic messages being used.

Change-Id: Icfd02b5f126892b1d5d2cebe8c8fb02b180000f7