encfs: upstream relocation and update to 1.7.5

* Upstream migrated to github
* Moved to previously semi official v1.7.5 which was released with backdating.
* Releases packed as is and now both head and stable require reconf.
* Introduce new dependencies.

Note:
New version creates vaults which are not backward compatible.
Older vaults are still working fine with new version.

This can be left as-is (`build.stable?`), right?