With that, (almost) only libvgz carries goto statements from zlib.

This isn't changing any of the previous semantics, in particular the
AN(be) assertion from the "ok:" jump is honored by all code paths
formerly leading to it.

Previously, the bitmap was allocated on the stack prior to the magic
check of shardd, which is now fixed at the expense of a potential code
style violation. But more importantly, we currently read the number of
backends prior to acquiring the read lock, but there is no evidence
that this was done on purpose and not overlooked, besides moving a
possibly expensive state initialization outside of the critical
section.

If that was on purpose, please document it.