Use a global R/W lock for configuration

Use a global R/W lock for configuration

Previously, we had one configuration mutex per module.
With a global read/write lock, resetting, loading, saving and
auto-saving the configuration becomes atomic (and use only one lock &
unlock pair). Also, multiple threads can now read the configuration
item of the same module at the same time.

Note that, as the earlier configuration mutex, only configuration item
values are protected. The list of items and their meta-data cannot
change while VLC runs (they're hard-coded in the plugin descriptors).