Enabling mute LEDs and mic mute LEDs

NOTE 1) Not sure if this needs a UDS session. Therefore I've labelled the blueprint correctly but not proposed it for the sprint.

NOTE 2) This is NOT about mute and mic-mute hotkeys, only about mute and mic-mute LEDs.

=== Current kernel status ===

Current kernel status:
 - For some HPs, the Mute LED is controlled through an alsa kcontrol called "Mute-LED Mode" that can be set to "Off", "On" and "Follow Master". Follow master is the default and means that it follows the state of the "master mute" control of the internal sound card. This is as of kernel 3.4 (IIRC, possibly 3.5).

 - For some Lenovos, the Mute LED and Mic-mute LEDs are controlled through sysfs, and the thinkpad-acpi driver. There is no deployed code today that makes use of this interface.

=== What could be done ===

 - We could discuss how we want the LED to behave. Should it always follow the internal card's mute/micmute status, or follow the current default card's mute/micmute status?

 - We could write a PulseAudio module that detects changes to the mute status for the default/fallback output (and input, for mic-mute) and sets the "mute-LED mode" control accordingly.

- For the thinkpad-acpi driver, an additional problem is that the currently logged in user can not access sysfs (only root can). As it happens, the thinkpad-acpi driver already creates a sound card, although I have never seen it having any correct function. We could piggy-back on that code and add a "Mute-LED Mode" and a "MicMute-LED Mode" control. This would both give a more uniform interface (i e same as for ), as well as solving the "currently logged in user" problem (sound cards are accessible to the logged in user).