Replace en_US.UTF-8 with C.UTF-8 in postgresql-check-db-dir
Suggest the C.UTF-8 locale that's shipped by default instead of en_US.UTF-8 which requires generating it before use.
Another advantage of C.UTF-8 is that it won't trigger the
collation version mismatch
warning after glibc or icu updates since
its collversion
is NULL.
Fixes #3 (closed)