* external.cc: Include new callout.h header.
(cygwin_internal): Initial implementation of CW_CALLOUT and CW_CYGHEAP_MALLOC.
(dll_preload): Handle CYGWIN=preload:foo.dll.
(dummy_callout): Default NULL callout function.
* globals.cc: Include new callout.h header.
(callout): Declare/define.
* uname.cc (uname): Initial implementation of callout handling.
* winsup.h (dll_preload): Declare.
* include/cygwin/version.h: Bump API minor number to 269.
* include/sys/cygwin.h: Define CW_CALLOUT, CW_CYGHEAP_MALLOC.