howto.html (GLIBCPP_FORCE_NEW): Document new environment variable which replaces all uses of __USE_MALLOC macro.

	* docs/html/23_containers/howto.html (GLIBCPP_FORCE_NEW): Document
	new environment variable which replaces all uses of __USE_MALLOC
	macro.
	* docs/html/ext/howto.html (GLIBCPP_FORCE_NEW): Likewise.
	(__mem_interface): Remove all references to old internal typedef.
	* include/backward/alloc.h (__USE_MALLOC): Remove it and all
	guarded code.
	* include/bits/c++config (__USE_MALLOC): Update related error
	message and comment.
	* include/bits/stl_alloc.h (__USE_MALLOC): Remove it and all
	guarded code.  Update all related comments.
	(__mem_interface): Unconditionally replace it with __new_alloc.
	However, leave the typedef around in case anyone used it.
	(__default_alloc_template<>::_S_force_new): New class static.
	(__default_alloc_template<>::allocate, deallocate): Add
	run-time controlled feature similar to what __USE_MALLOC code
	path had provided.
	* src/stl-inst.cc (__USE_MALLOC): Remove it and all
	guarded code.
	* testsuite/21_strings/capacity.cc: Remove reference to __USE_MALLOC.
	Add documentation on GLIBCPP_FORCE_NEW environment variable.
	* testsuite/ext/allocators.cc: Likewise.

From-SVN: r58286