samba: bump 3.6.18 -> 3.6.20 (security fixes)

3.6.20 is a security release in order to address CVE-2013-4475 (ACLs are
not checked on opening an alternate data stream on a file or directory)
and CVE-2013-4476 (Private key in key.pem world readable).

And 3.6.19 had some bug fixes:
http://www.samba.org/samba/history/samba-3.6.19.html