attempt to repair field unicity feature; see #16

[btry]

• add linkfield to search options only when needed
• this makes the argument on PluginGenericobjectObject::getobjectSearchOptions() useless
• re-enable the checkbox to enable field unicity
• reorder search options by key (very important)

[btry]

FYI see this proposal from a contributor, very close to this PR

https://github.com/pluginsGLPI/genericobject/issues/66#issuecomment-178066405

[trasher]

Is this one a feature or a bugfix?

[btry]

This pr restores and fixes a a disabled feature because it was buggy. Then I woukd say ... Both !

[trasher]

If the feature was not enabled in lasts releases; this is a feature, not a bugfix; and then it should reach develop IMO.

[btry]

OK for me