plugin_vars_free_values() was walking plugin sysvars and thus
did not free memory of plugin PLUGIN_VAR_NOSYSVAR vars.

* change it to walk all plugin vars
* add the pluginname_ prefix to NOSYSVARS var names too,
  so that plugin_vars_free_values() would be able to find their
  bookmarks

Making changes to wsrep_mysqld.h causes large parts of server code to
be recompiled. The reason is that wsrep_mysqld.h is included by
sql_class.h, even tough very little of wsrep_mysqld.h is needed in
sql_class.h. This commit introduces a new header file, wsrep_on.h,
which is meant to be included from sql_class.h, and contains only
macros and variable declarations used to determine whether wsrep is
enabled.
Also, header wsrep.h should only contain definitions that are also
used outside of sql/. Therefore, move WSREP_TO_ISOLATION* and
WSREP_SYNC_WAIT macros to wsrep_mysqld.h.

Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>

look for an installed plugin with the same name _and the same type_
(in case there are many plugins with the same name and different type,
which is, technically, possible for built-in plugins).