This commit adds directory monitor for the Installed Packages
Repository in order to catch, through dnofity, repository change
events and signal repositories_updated() once locks are released.
Given the current Python limitations in supporting dnotify without
getting dirt, DirectoryMonitor dispatches SIGIO to the callback
for both installed repository directory change and RigoDaemon
executable. It is up to the code determine what effectively changed.
Moreover, enabling DN_MULTISHOT causes the signal handler to be
kept alive until the dnotified file descriptors are explicitly closed
(DirectoryMonitor.close()). This is of course sub-optimal because
we get called more than needed, but still much better than polling.
Fabio Erculiani
890b447785