samba4: security bump to version 4.7.6

samba4: security bump to version 4.7.6

CVE-2018-1050: Vulnerability to a denial of service attack when the RPC
spoolss service is configured to be run as an external daemon.

https://www.samba.org/samba/security/CVE-2018-1050.html

CVE-2018-1057: Authenticated users might change any other users'
passwords, including administrative users and privileged service
accounts (eg Domain Controllers).

https://www.samba.org/samba/security/CVE-2018-1057.html

Add clnt_create() test result to cache to fix cross configure breakage.

Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>