MDEV-14580: mysql_install_db elements based on dirname of mysql_instal_db

[grooverdan]

tested with the following with cwd at builddir:

$ make DESTDIR=/tmp/m3 install
$  /tmp/m3/usr/local/mysql/scripts/mysql_install_db --datadir=/tmp/data_fullpath
$ mv /tmp/m3 /tmp/m4
$ /tmp/m4/usr/local/mysql/scripts/mysql_install_db --datadir=/tmp/data_moved
$ export PATH=/tmp/m4/usr/local/mysql/scripts:$PATH
$  mysql_install_db --datadir=/tmp/data_path
$ cd  /tmp/m4/usr/local/mysql/scripts
$ ./mysql_install_db --datadir /tmp/relative

I submit this under the MCA.

[vuvova]

I've merged it into 5.5 (bb-5.5-serg for now) with a follow-up commit to remove additional dependencies. The history taught me that mysql_install_db is quite brittle, so I'd try to avoid adding anything that it doesn't use now. I've replaced $(...) syntax with backticks and added some safety checks in case dirname isn't available.

Hope it's ok.

[grooverdan]

fine. looks good. thanks for the merge.