Add event queue handling. It triggers activities on events read from

Add event queue handling. It triggers activities on events read from
/dev/usb. The actions are specified in the file /etc/usbd.conf.

usbd.c:
   - Add event queue (/dev/usb) handling.
   - Add comments
   - Clean up code some more

usbd.8:
   - Update manpage for the new command line flags
   - Remove a duplicate FreeBSD tag from it).

usbd.conf, usbd.conf.5, Makefile:
   - Add the usbd.conf configuration file and the man page for it.

NOTE: MAKEDEV already creates the /dev/usb device tree node, no change
needed there anymore.