Until now, the RPC framework did not support const RPC functions. Rather
than being a limitation inherent to the concept, const RPC functions
plainly did not exist. So supporting them was not deemed too important.
However, there are uses of RPC interfaces that would benefit from a way
to declare an RPC function as const. Candidates are functions like
'Framebuffer::Session::mode()' and 'Input::Session::is_pending()'.
This patch clears the way towards declaring such functions as const.
Even though the patch is simple enough, the thorough support for
const-qualified RPC functions would double the number of overloads for
the 'call_member' function template (in 'base/include/util/meta.h'). For
this reason, the patch does support const getter functions with no
arguments only. This appears to be the most common use of such
functions.
