Implement getcontext, setcontext, makecontext, swapcontext

Implement getcontext, setcontext, makecontext, swapcontext

        * common.din (getcontext): Export.
        (makecontext): Export.
        (setcontext): Export.
        (swapcontext): Export.
        * exceptions.cc (__unwind_single_frame): New static functions, 64 bit
        only.
        (setcontext): New function.
        (getcontext): New function.
        (swapcontext): New function.
        (__cont_link_context): New function.
        (makecontext): New function.
        * include/cygwin/version.h (CYGWIN_VERSION_DLL_MAJOR): Bump to 2002.
        (CYGWIN_VERSION_API_MINOR): Bump.
        * include/ucontext.h (getcontext): Add prototype.
        (setcontext): Ditto.
        (swapcontext): Ditto.
        (makecontext): Ditto.
        * ntdll.h (NtContinue): Ditto.

        * new-features.xml (ov-new2.2): Add new section.  Document getcontext,
        setcontext, makecontext, swapcontext.
        * posix.xml (std-deprec): Add getcontext, setcontext, makecontext,
        swapcontext.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>