Add speaker device

[vinc]

This PR will add /dev/speaker allowing the beep command to interact with the PC Speaker from usermode.

Writing a (big endian) bytes representation of a 64 bit float to this device will start to play a sound at the given frequency or stop it if the frequency is zero.

[vinc]

If we write the frequency to the device using a string instead of a float we can use the shell more easily to play sound:

And the beep command could be a simple script that uses print to write the frequency to the device, then sleep to wait for the sound to play, then print again to stop the sound:

print $1 => /dev/speaker
sleep $2
print 0 => /dev/speaker

[vinc]

The beep command can now become a regular userspace program.