Recommendation is to avoid using the year nomenclature as this
information is already encoded in the git repo. Avoids needing to
repeatly update.
Also updates AUTHORS.txt from current repo with contributor names
This function is called only from three sites, it doesn't need to be in
common.h.
The implementation was already in systemdirsappend.cpp, so a matching
header was created for it.
This reduces the visibility of the function to only the files that need
it and slightly shrinks common.h.