enhance r_debug.r_brk/link.h/_dl_debug_state

[John Reiser]

It would noticeably help the development of in-process debuggers
and monitors to enhance elf/link.h so that r_debug.r_brk changes from
    ElfW(Addr) r_brk;
to
    void (*r_brk)(struct r_debug *);

with the calls in elf/rtld.c, elf/dl-init.c, elf/dl-close.c
changing from
    _dl_debug_state();
to
    (*_r_debug.r_brk)(&_r_debug);

and the definition of _dl_debug_state() becoming
    void
    _dl_debug_state(struct r_debug *r)
    {
        (void)r;  /* do nothing */
    }

Such changes would make it much easier for an in-process monitor
to receive notice of state changes, because it would not be
necesary to plant a breakpoint on _dl_debug_state and handle
SIGTRAP, or to use other devious means.  Instead, the monitor
could just point DT_DEBUG.r_brk at its own procedure.

-- 
John Reiser, address@hidden