Enforce that solo5_{block,net}_acquire() may only be called once

[mato]

(...)

Having said that, I can see how _acquire_ implies a transition of some kind, so perhaps it'd be better to just change the docs (solo5.h) to say that you're only allowed to call it once per handle?

Originally posted by @mato in #402

[mato]

This is partially addressed by documenting the intended behaviour in solo5.h in #407. Actually enforcing this for hvt and spt is surprisingly hard due to the bindings-owned manifest structures being read-only, so leaving this open for dealing with in a future iteration/re-evaluation of how the manifest structures are dealt with.