plans for include/c subdirs

[Benjamin Kosnik]

The current include directories for "C" headers exist:

include/c/		"C" bits in global, standard-bits using std::foo
include/c_std/ 		shadow header work

As part of the evolution of the "C" header compatibility work, I'm
proposing the following:

include/c/		include "C" headers, no changes, no using decls
include/c_std/		"C" bits in global, standard-bits using std::foo
include/c_shadow/ 	shadow header work

this seems to make more sense from a naming point of view, and it has
the added benefit that when/if __cplusplus gets re-defined to the
correct value, things like solaris2.8 will work if they use the
include/c/ directory. 

If there are no problems with this, I'll do it later tonight.

-benjamin