site: Explicitly track name resolution status

Introduce a new variable st->resolving which tracks whether we have an
outstanding name resolution request.  This makes it safe to (try to)
start name resolution (via the new function ensure_resolving) multiple
times etc.

No resulting functional change from just this patch.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
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Changes in v2:
  * Do slightly complicated dance with st->resolving, which is needed
    because of the reentrancy hazard posted by resolver->request.  In
    v1 of the series there was a bug here which could cause the site
    state machine to lock up.