libcli/auth: add netlogon_creds_cli* infrastructure

libcli/auth: add netlogon_creds_cli* infrastructure

This provides an abstraction to hide netlogon_creds_CredentialState,
which is stored in a node local tdb.

Where the global state (netlogon_creds_CredentialState) between client and
server was only kept in memory (on the client side), we now use
the abstracted netlogon_creds_cli_context.

We now use a node specific computer name in order to establish
individual netlogon sessions per node.

If the caller wants to use some netlogon calls with credential chain
(struct netr_Authenticator), netlogon_creds_cli_lock*() is used
to get the current netlogon_creds_CredentialState in a g_lock'ed
fashion, a talloc_free() will release the lock.

The locking is needed as there might be more than one process
(multiple winbindd child, cmdline tools) which want to talk
to a specific domain controller. The usage of netlogon_creds_CredentialState
needs to be serialized as it uses sequence numbers.

LogonSamLogonEx doesn't use the credential chain, but for some operations
it needs the global session in order to de/encrypt individual fields.
It uses the lockless netlogon_creds_cli_get() and netlogon_creds_cli_validate()
functions, which just make sure the session hasn't changed between
get and validate.

This is prepares the proper fix for a large number of bugs:
https://bugzilla.samba.org/show_bug.cgi?id=6563
https://bugzilla.samba.org/show_bug.cgi?id=7944
https://bugzilla.samba.org/show_bug.cgi?id=7945
https://bugzilla.samba.org/show_bug.cgi?id=7568
https://bugzilla.samba.org/show_bug.cgi?id=8599

Signed-off-by: Stefan Metzmacher <metze@samba.org>