The documentation for directory_update_init() was incorrect: a job ID
must be positive, not non-negative. If the update queue is full and
no job was created, it makes more sense to return 0 instead of -1,
because it is more consistent with the return value of isUpdatingDB().
If the user requests database update during startup, call
directory_update_init(). This should be changed to fully asynchronous
update later.
For this to work, main_notify has to be initialized before db_init().
exploreDirectory() duplicates some code in updateDirectory(). Merge
both functions, and use directory_is_empty() to determine whether
update or explore mode should be used.
The source directory.c mixes several libraries: directory object
management, database management and database update, resulting in a
1000+ line monster. Move the whole database update code to update.c.
