Lower the priority of the thread so forked-daapd may still respond
during file scan on low power devices
Remove EVLOOP_ONCE calls in filescanner
These calls generate error messages when the loop is already running, which
will be the case when using init-rescan or full-rescan. The only purpose
of these calls seems to be to check for exit signals from the main thread
during a startup bulk scan, where the loop is not running yet. However, we
can check for an exit signal by just setting/checking scan_exit.

This commit also removes the unused filescanner_status().
Don't try to login to Spotify if the filescanner got an exit signal
Add a function in filescanner.c to enumerate certain file types
Fixes bug where init-rescan and full-rescan would run twice because
two inotify events get triggered by eg 'touch xxx.init-rescan'
Fix filescanner bugs:
- stop init-rescan/full-rescan from running twice
- set thread priority to batch
- don't request libevent to loop when the loop is already running