Add BSD fallback for syspaths.CONFIG_DIR

In the absence of _syspaths.py, make sure that the BSDs still look for
configuration files in /usr/local/etc/salt.
Make platform variable private

This makes it so that platform is not exposed as syspaths.platform.
Merge pull request #11049 from terminalmage/bsd-syspaths-fallback-2014.1

Add BSD fallback for syspaths.CONFIG_DIR (2014.1 branch)