plugins: Do not ever unload a plugin after calling into it

plugins: Do not ever unload a plugin after calling into it

This is what can happen in a plugin_init function:
- An element based on GstBaseSink is registered
- Other elements fail to register
- The plugin_init function returns FALSE

Now if this the plugin is the first plugin to link against
libgstbase.so, it will have caused libgstbase.so to be loaded and static
strings from that library will have been added to gobject while
registering GstBaseSink.

So unloading the plugin will cause those strings to go stale and the
next plugin using GstBaseSink will crash. So we must not unload modules
after calling into them ever.

https://bugzilla.redhat.com/show_bug.cgi?id=572800