non-shadowed "C" headers, reworked, cleaned

[Benjamin Kosnik]

I just finished cleaning up the non-shadowed "C" headers. As part of
this, I removed all the gnu-linux-isms in the header files, all the
hacky defines and whatnot (and moved these into
config/os/gnu-linux/bits/os_defines.h).

I also put select functions and types into namespace std::, as
required by the standard. This might break a few platforms (I test on
RH 6.2, RH 7.0, x86 and powerpc) or different os's: please let me know
if you see breakage.

The solution for breakage is to add a check for the function in
autoconf, and then add the correct _GLIBCPP_HAVE_XXX in the
include/c/bits/std_cfoo.h header. 

To help everybody out, here's a rundown on the current "C header solutions:
include/c: "C" header names in global, names for C++ conformance in std
include/c_std: "C" header names in _C_legacy, names for C++ conformance in std

Of the two, include/c is working.

thanks,
benjamin