Share ACLs and EGroup-Ownership in EOS

<!--HTML-->To consolidate the concept of sharing implemented inside EOS for any access protocol we are currently adding a new type of ACL which defines a 'share'. One of the new characteristics of a share ACL is that they are not influenced by POSIX or classic ACLs. We support additional ACL capabilities as 'can share'. A second important new concept is the concept of ownership by an EGROUP. Ownership by individuals is problematic when people depart from CERN. This requires often a manual change of ownership of a departed person if files reside in shared spaces. An EGROUP ownership is beneficial in particular in shared areas like project spaces, which are currently soved by the creation of one service account per project.