unwind support for Linux 2.6 vsyscall DSO

[Roland McGrath]

> SOLIB_ADD will only be called when the dynamic linker's "hey, I've
> just (un)loaded something new" breakpoint has been hit.  (More
> precisely, this is the breakpoint placed at r_brk in the r_debug
> struct.) This occurs quite early in the execution of the program, but
> it might not be early enough.  Also, as Daniel alludes, it won't
> happen at all for static binaries.

Ok, then that is really not sufficient.  I apparently got a mistaken
impression of how early it gets called from the previous discussion.  The
more I think about it, the more a single new hook that is clearly "inferior
address space we haven't seen before" (i.e. exec/attach but not fork) looks
like the clean and sensible thing.  All the existing hooks have semantics
that are either not quite clear or are clearly different from that.