Core: Fixed set method in UF:V:shell module.

Set method calls itself in old behavior, so it causes stack overflow.
In new version this method calls set on UF:D:shell attribute.
Core: fixed behavior of module UF:D:shell

UF:D:shell can be null now because shells are selected primary by
U:D:preferredShells. UD:F:shells is used for forcing one shell on
selected facility for the user. For the same reason fill method is no
longer required (and wanted) for UF:D:shell.
Merge pull request #36 from licehammer/shellsModules

Core: Merge fixes in shells modules