component/mariadb: install in shared mode

Mariadb was not shared, because installing mroonga writes a plugin in
mariadb's plugin dir and it's not allowed for one part to write in
another part's folder.

The approach is to install mroonga plugin in it's own plugin directory,
then copy all mariadb default plugins in this plugin directory and
configure instance to use mroonga's plugin directory.

Groonga also has plugins and we are using groonga-normalizer-mysql
plugin. Fortunately, groonga reads plugins located in paths listed in
GRN_PLUGINS_PATH environment variable, so we can use a simpler approach
of installing plugins in their own installation folder and set
GRN_PLUGINS_PATH in the environment of processes using groonga, ie. the
mariadb server process.