Remove dead support for changing BDB hash algorithm and cache of alternatives

ok natano@ millert@ deraadt@

Move the last of the __DBINTERFACE_PRIVATE bits from <db.h> to libc's wrapper
and eliminate the now superfluous -D option

ok kettenis@ millert@

Pull in namespace.h when building all .c files using gcc's -include option,
so that we can provide asm labels for the memcpy/memset/__stack_smash_handler
calls that it generates ab initio. Eliminate

Pull in namespace.h when building all .c files using gcc's -include option,
so that we can provide asm labels for the memcpy/memset/__stack_smash_handler
calls that it generates ab initio. Eliminate direct #includes of it. Make
sure it's a dependency of all objects (unnecessary for asm, but close enough).

ok deraadt@

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Use new framework for wrapping dbopen()
Move internal declarations from <db.h> to libc's hidden/db.h

ok kettenis@