glibc: Hurd dynamic linker fixes

[Roland McGrath]

We very much appreciate all the time and effort you have put into this.
Unfortunately, I (the prime Hurd libc expert) have very little time right
now to examine changes or debug Hurd and libc things.  Many of your changes
have seemed undesireable to me, but I have not had the time to examine them
all in sufficient detail to explain why or suggest better solutions, and I
won't have much more time any time soon.  It would be very helpful if you
could explain what problems you encountered and the rationale for how you
are trying to solve them, rather than just sending large patches that make
a lot of changes with no real explanations in the log entries or comments.