new directive: proxy_next_tries N

[duv]

Short description: Will attempt only N upstreams and then fail with last
error

I didn't commit this to any trunk/branch,
This is my first patch so I'm not sure how it goes. 
I submitted a gist of it to:
https://gist.github.com/shai-d/5446961

Hope you will find it useful.
Shai Duvdevani.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,238589,238589#msg-238589