channelmap: Add 2.1 surround

Add surround-21 as a alias to set up a 2.1 channel map.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
alsa-mixer: Add surround 2.1 profile

Surround 2.1 is one of the more common surround profiles these days,
so it's about time we support it.

The "surround21" was added to alsa-lib a few months ago, and there
hasn't yet been an alsa-lib release since, but I doubt it will change.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
po: Add two files to POTFILES.in

Translations were recently to these to files.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
bluetooth: Rename variable to improve code readability

The label 'u' is used throughout the code to name pointers to the struct
userdata.
bluetooth: Notify the main thread of a stream fd HUP