scanner: Clean up non-destructive rescan

Fix several issues with the non-destructive rescan functionality.
Most of these issues also affected inotify scanning as well.  These
include annoying debug messages, adding album art for files that we
aren't supposed to be scanning anyway, incrementing the UpdateID when no
changes were made to the database, and other smaller issues.

upnpglobalvars.c

@@ -85,8 +85,6 @@ char db_path[PATH_MAX] = {'\0'};
 char log_path[PATH_MAX] = {'\0'};
 struct media_dir_s * media_dirs = NULL;
 struct album_art_name_s * album_art_names = NULL;
-short int scanning = 0;
 volatile short int quitting = 0;
 volatile uint32_t updateID = 0;
 const char *force_sort_criteria = NULL;
-short int rescan_db = 0;