* cxx.cc (default_cygwin_cxx_malloc): Enhance commenting.

	* cxx.cc (default_cygwin_cxx_malloc): Enhance commenting.
	* dll_init.cc (dll_dllcrt0_1): Likewise.
	* dlfcn.cc (dlopen): Prevent dlopen()'d DLL from installing any
	cxx malloc overrides.
	* include/cygwin/cygwin_dll.h (__dynamically_loaded): New variable.
	* lib/_cygwin_crt0_common.cc (_cygwin_crt0_common): Check it and only
	install cxx malloc overrides when statically loaded.  Extend comments.