gst/gst.c: Make init_pre and init_post take the full complement of GOptionFunc args...

gst/gst.c: Make init_pre and init_post take the full complement of GOptionFunc args so they can return useful GErrors...

Original commit message from CVS:
* gst/gst.c: (init_pre), (scan_and_update_registry),
(ensure_current_registry_nonforking),
(ensure_current_registry_forking), (ensure_current_registry),
(init_post), (gst_debug_help), (gst_deinit):
Make init_pre and init_post take the full complement of GOptionFunc
args so they can return useful GErrors. Make the registry updating
functions do so.
Call _priv_gst_registry_remove_cache_plugins after scanning files to
ensure that the registry we're about to write out doesn't contain
stale information about old-deleted plugin files.
Make _priv_gst_registry_remove_cache_plugins return a boolean so
that deletion of plugin files is considered a registry change.
* gst/gst_private.h:
* gst/gstregistry.c: (gst_registry_finalize),
(gst_registry_remove_features_for_plugin_unlocked),
(gst_registry_remove_plugin), (gst_registry_scan_path_level),
(gst_registry_scan_path),
(_priv_gst_registry_remove_cache_plugins),
(_priv_gst_registry_cleanup):
* gst/gstregistry.h:
Rename _gst_registry_remove_cache_plugins and _gst_registry_cleanup
by adding _priv prefix, so that they won't appear in the global
symbol table. They still do atm though because of #318031. Move the
prototypes to gst_private.h
When removing a plugin, remove all features for that plugin too.
Fixes #340878.