PSA: Formalizing the glibc<->gdb probe-based interface.

I don't plan to fix this right now, but I thought I'd kick off
a discussion on the topic so I can go back to this when we think
about fixing it.

All we have in glibc right now is a text file:
elf/rtld-debugger-interface.txt

We should promote this to the glibc manual and document
the interface as something for which we provide ABI promises
e.g. the probes won't go away.

We document *other* probes in manual/probes.texi, but we should
add a section for stable guaranteed probes, and discuss a way
to deprecate the probes should we ever need to do that.

-- 
Cheers,
Carlos.