Rename sampgdk_plugin_find_symbol to *_get_symbol

Since the rest of the "plugin" functions are "get" and the corresponding
public API is called "sampgdk_get_plugin_symbol", I thought it would be
better to rename this for consistency.
Fix a bug in sampgdk_plugin_unregister()

sampgdk_plugin_unregister() was mistakenly setting the head of the plugin
list to NULL if called on the last registered plugin. This essentially
could pop all remaining plugins off the list!
Fix bad params pointer passed to public hook