unsetenv(3), envlength(3), getenvindexed(3), and environ(7).
This provides the user-space foundation for environmental variables.
Note that this works over fork(2), but not execve(2) yet.
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.