Remove obsolete compatibility function declarations for snprintf() and vsnprintf()

We were providing function declarations without ever providing function implementations.
Also, for some reason, this caues compilation issues on older Linux versions.
Set version to 3.0.6
Bump ABI version and include ABI file for 3.0.5 and 3.0.6
Update release notes for 3.0.6
Merge branch 'branch-netatalk-3-0' into release-3.0.6