ENH: various dlLibraryTable improvements/refinements (#1737)

- libs() singleton method for global library handling

- explicit handling of empty filename for dlLibraryTable open/close.
  Largely worked before, but now be more explicit about its behaviour.

- add (key, dict) constructor and open() methods.
  More similarity to dimensionedType, Enum etc, and there is no
  ambiguity with the templated open().

- construct or open from initializer_list of names

- optional verbosity when opening with auxiliary table,
  avoid duplicate messages or spurious messages for these.

- basename and fullname methods (migrated from dynamicCode).

- centralise low-level load/unload hooks

- adjust close to also dlclose() aliased library names.