command: format ${avsync} like on terminal status line

Padding with spaces is very useless for OSD (because most fonts are
variable width), but it's good when using it on the terminal, e.g. for
reproducing the default terminal status line.
manpage: ao_alsa: link upmix wiki entry

Also clarify the statement about what we expect to happen by default.
It's well possible that distros at some point will fix their ALSA
configuration, and e.g. enable the upmix plugin by default.
build: require alsa libs not older than 6 years

Some IEC958 flags we use have been introduced in 2008, which makes
compilation fail on older systems.
vo/w32_common: use local definition of IID_ITaskbarList2

It is not defined in the current cygwin release.
Revert "vo/w32_common: use local definition of IID_ITaskbarList2"

This reverts commit 17067343eb10293beb61d3811680b9c59f4db311.

Embarassingly, this turned out not to be necessary.
ao/wasapi: use IsEqualGUID and IsEqualPropertyKey

before we were reinventing this wheel
ao_coreaudio: add missing goto for error path
ao_coreaudio: fix mono/stereo channel mapping

Needed after af3bbb800d since now we use channel mapping all the time.

Fixes #1357