alsa-lib: Bug fixes to namehint, dsnoop and minor changes

[Mark Hills]

I've been running these patches in some form for a while. Now tidied up 
for RFC.

It's quite possible I misinterpreted something; the behaviours seem to be 
quite long-standing, hence a first pass for comments.

This has taken me to the _avail() functions, which do not seem to be 
prepared for signed output, yet callers interpret as signed and ultimately 
end up in external API. Their handling of wraparound conditions etc. may 
not be as expected. I've already started to look at these.

-- 
Mark