Intermediate commit.

Fix engine list lifetime for sys_var_pluginlist.

The Sys_var class assumes that some operations can be done without
explicitly freeing resources, for example default_value_ptr(). Thus,
methods (like Sys_var_pluginlist::do_check) need to generally work
with temporary lists, which are registered in the THD to be
freed/unlocked automatically. And do_update() needs to make a
permanent copy to store in the global variable.