Set proper fallback CONFIG_DIR for NetBSD

This is not a critical fix since the official package for NetBSD uses
--salt-config-dir to set the config dir. But, since NetBSD uses pkgsrc,
which defaults to /usr/pkg/etc, a NetBSD-specific fallback should be
set.

See https://github.com/saltstack/salt/issues/11889#issuecomment-40500747
for more information.
Merge pull request #12014 from terminalmage/netbsd-syspaths

Set proper fallback CONFIG_DIR for NetBSD