STYLE: IOobject/regIOobject - noexcept methods, isolate local functions

- local writeHeaderEntry helper was not marked as file-scope static.

- use do/while to simplify handling of padding spaces

ENH: IOobject - copy construct, resetting name and local component

- when copying with a new local component, this is simpler than
  constructing from all of the components, which was previously the
  only possibility for setting a new local component.