Moving alias expansion yet again - this time into AliasManager itself

We need alias handling both client (for /exec) and core (for perform) side.
Thus, we need it in mod_common. AliasManager is now able to process an input
line, providing a list of resulting commands. In doing this, it also ensures that
privmsgs are always prefixed by /SAY.
The client and core input handlers both pipe commands through AliasManager now. At
some point, we can think about client-only and core-only aliases, maybe.