before running mysql_install_db check for the existence
of $datadir/mysql, not simply $datadir ($datadir might be mounted on
a separate device - exists, but empty)
cmake/cpack_rpm.cmake:
* mark all cnf files with %config(noreplace)
* add the forgotten postun script
sql/sys_vars.cc:
0 for a string variable means "no default. But datadir has the default value.
support-files/rpm/server-postin.sh:
* use mysqld --help to determine the correct datadir in the presence of my.cnf files
(better than my_print_defaults, because it considers the correct group set).
* Only create users, and chown/chmod if it's a fresh install, not an upgrade.
* only run mysql_install_db if datadir does not exist
