logging: site: Log state on PHASE_RUN entry instead of initially

logging: site: Log state on PHASE_RUN entry instead of initially

site_startup calls enter_state_run which would print a message, but
logging is not set up that early.  The result is a message printed to
stderr before daemonisation.

We can distinguish this situation from other calls to enter_state_run
because the old state is SITE_STOP, which only occurs between config
reading (closure invocation) and site_startup being called.
So we can suppress this message.

But it did serve a purpose: it would only be printed if the site was
listed in `sites'; otherwise site_startup wouldn't be called and the
`entering state RUN' message would be absent.

So instead we provide a more explicit way to tell: on entering
PHASE_RUN, site_startup has either been called, or not.  And logging
is set up.  state is then STOP or RUN.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>