get rid of the MULTI_KBOARD #ifdefs

[Dan Nicolaescu]

src/config.in contains an unconditional #define for MULTI_KBOARD. 
The Windows port also defines it unconditionally in s/ms-w32.h
The Mac Carbon port is not functional anyway, but it ought to define
it too.

So I think we should get rid of all the #ifdef MULTI_KBOARD
conditionals and just leave the code for when MULTI_KBOARD is
defined, this will help reduce the cruft in the code base. 

RMS OKed this provided there no objections here...

Comments?