installed on debug version of server

When starting a server using mysqld_safe with the option
--mysqld-version=debug, the debug version of the server
is started. When attempting to load a plugin that is not
debug-built, the load will fail because a non-debug built
plugin use, e.g., fastmutex while the debug-built server
uses safemutex.

To solve this problem, mysqld_safe will set the plugin-dir
to use a directory where debug-built plugins are placed.

This patch sets the plugin dir to <plugin-dir>/whatever,
when mysqld-version=whatever, and just <plugin-dir> otherwise.