samba: revert to 3.6.8

samba 4.0.0+ doesn't reliably compile on Snow Leopard. In
addition, the autoconf build used by our 4.0.0 formula only
installs a subset of samba's tools. (See the readme for details.)
See #17820.

Once these issues are resolved, we can upgrade to 4.0.3.

This reverts commit 2229c7d9bb04bb523c9d103c5d8f2e9a60098ee5.