Samba CVE-2016-2114: "server signing = mandatory" not enforced

Due to a regression introduced in Samba 4.0.0, an explicit "server signing = mandatory" in the [global] section of the smb.conf was not enforced for clients using the SMB1 protocol.

As a result it does not enforce smb signing and allows man in the middle attacks.

This problem applies to all possible server roles: standalone server, member server, classic primary domain controller, classic backup domain controller and active directory domain controller.

In addition, when Samba is configured with "server role = active directory domain controller" the effective default for the "server signing" option should be "mandatory".

During the early development of Samba 4 we had a new experimental file server located under source4/smb_server. But before the final 4.0.0 release we switched back to the file server under source3/smbd.

But the logic for the correct default of "server signing" was not ported correctly ported.

Note that the default for server roles other than active directory domain controller, is "off" because of performance reasons.