core: use opt_incognito in a few more applog() places.
pool: abstract get_pool_name()

The introduction of get_pool_name() warrants a separate function
so opt_incognito does not litter everywhere.
pool: define and use POOL_NAME_INCOGNITO.
pool: introduce get_pool_user() for simpler opt_incognito use.

Use immediately.
pool: use get_pool_user().
pool: use get_pool_name() where appropriate.
pool: do not use defines, just inline strings.
pool: display URL instead of "Pool N" if no poolname provided.

This works since get_pool_name() in pool.c checks if an empty string is
provided, and uses the URL instead if so.