[v3] -Wshadow fixes

[Benjamin Kosnik]

Last week I tangled with a bugzilla full of Red Hat users complaining
about -Wshadow and boost. I think a pre-emptive cleanup of the libstdc++ 
headers is a good idea, which will prevent all the other libstdc++
maintainers from having to do this in the next couple of months...

tested x86/linux
gcc
gcc-4_2-branch
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