Improved devfs_install

[rafalcieslak]

This branch adds an optional DEVFS_INSTALL_FLAG_NUMBERED flag for devfs_install, which allows installing multiple devices sharing the same name into /dev - they will be automatically numbered e.g. /dev/vga (== /dev/vga0), /dev/vga1, /dev/vga2, ... This removes the silly assumption that only the first device of a particular driver may expose a /dev interface.

I've also fixed a potential race condition in devfs_install.

[cahirwpz]

I've got completely different view on device naming scheme. Name and unit number of a device, according to my understanding of *BSD device drivers, comes from devclass structure. Each device is assigned devclass after it's created and device.nameunit is set to unique identifier (rtc0, video1, tty7, etc.) by calling devclass_add_device.

[cahirwpz]

Obsoleted by #344. With addition of device classes (incl. device->d_nameunit field) and rewrite of devfs using (yet to be written) pseudo filesystem framework, we won't need this patch.