The stub status module and ngx_http_send_response() (used by the empty gif
module and the "return" directive) incorrectly assumed that responding
to HEAD requests always results in r->header_only being set.  This is not
true, and results in incorrect behaviour, for example, in the following
configuration:

   location / {
       image_filter size;
       return 200 test;
   }

Fix is to remove this incorrect micro-optimization from both stub status
module and ngx_http_send_response().

Reported by Chris Newton.

No functional changes.

The "stub_status" directive does not require an argument.

This fixes content type set in stub_status and autoindex responses
to be usable in content type checks made by filter modules, such
as charset and sub filters.

The module always produces only one and obviously the last buffer in chain.

And corresponding variable $connections_waiting was added.

Previously, waiting connections were counted as the difference between
active connections and the sum of reading and writing connections.
That made it impossible to count more than one request in one connection
as reading or writing (as is the case for SPDY).

Also, we no longer count connections in handshake state as waiting.

Three new variables were added: $connections_active, $connections_reading
and $connections_writing.