sfork is much like rfork except sharing is default for everything.
Eventually, I'll make a rfork(3) wrapper function around sfork(2) to
provide compatibility to BSD programs.
I don't like Linux clone(2): that's some messy function.
C and C++ files are now kept together and so are the mxmpp declarations.
Header files are now stored in include/ and mxmpp'd into preproc/.
All other code now -I ../libmaxsi/preproc.
And other stuff to make this happen, including refactoring Makefile.