Implement LWG 2192 and LWG 2294 for std::abs

Implement LWG 2192 and LWG 2294 for std::abs

	* doc/xml/manual/intro.xml: Document LWG 2192 changes.
	* doc/html/*: Regenerate.
	* include/Makefile.am: Add bits/std_abs.h.
	* include/Makefile.in: Regenerate.
	* include/bits/std_abs.h: New header defining all required overloads
	of std::abs in one place (LWG 2294).
	* include/c_global/cmath (abs(double), abs(float), abs(long double)):
	Move to bits/std_abs.h.
	(abs<_Tp>(_Tp)): Remove.
	* include/c_global/cstdlib (abs(long), abs(long long), abs(__int<N>)):
	Move to bits/std_abs.h.
	* testsuite/26_numerics/headers/cmath/dr2192.cc: New test.
	* testsuite/26_numerics/headers/cmath/dr2192_neg.cc: New test.
	* testsuite/26_numerics/headers/cstdlib/dr2192.cc: New test.
	* testsuite/26_numerics/headers/cstdlib/dr2192_neg.cc: New test.

From-SVN: r240660