add proxy_connect_bind directive for proxy_connect module

It's very useful to bind different IP when do forward proxying, for
crawler like applications. With ngx_lua's help, nginx can be used as
connection manager seriously.

Copying ngx_http_upstream_get_local() from http/ngx_http_upstream.c
shamelessly. It's better to remove 'static' from
ngx_http_upstream_get_local(), so we can use it directly. But don't
touch it at this time.