mail: add mailhidden attribute

When this attribute is enabled its value is "YES" otherwise it is "NO".

Purpose of this attribute is to hide mail addresses in addressbooks.
This attribute can be applied on a user or group.
Merge branch 'mailhidden' into mds-2.6
radius: new module
mds: bump version to 2.5.77 (RC)
samba4: sync Administrator
core: Adds unit file for systemd
samba4: Fixes dependencies for s4sync service