A little bit of sanity to dbiNew()

A little bit of sanity to dbiNew()
- lose the hysterical dbi_use_env condition and usedbenv option
  this is always enabled
- ensure mpool is always initialized, lose the config option (this
  is a mandatory BDB subsystem, not a bleeping configurable)
- let pagesize be what it is even when no configuration is present
- avoid overriding dbi_mmapsize and dbi_cachesize if set in the configuratio
- these are all per-environment, not per-dbi settings but for now...