Merge changes from ngx_http_scgi_module 1.0.15.

The merge_params stuff have not been incorporated because they're just refactoring.
Support Nginx 1.3.15 HTTP protocol switching API.
Merge changes from ngx_http_scgi_module 1.2.0.

The following things have not been incorporated:

 * ngx_http_upstream_param_set_slot
 * ngx_http_script skip_empty support.
Merge changes from ngx_http_scgi_module 1.2.7.
Merge changes from ngx_http_scgi_module 1.3.15.