Fixes #1693.

It might contain non-printable chars.

matter what protocol, a request is quite likely to happen soon-ish.

(for lack of any better place) since they will become involved in
protocol switching.

Complain about unrecognized protocols.

Presently we have two switch points:

  Created/Waited session and
  Rescheduled (from waiting list) request.

In the future we will need to tell what protocol controls a give
session, for instance when we come back from the waiter or waiting
list.   Rather than spend memory on a dedicated field for this,
protocols will share the enum sess_step, and ranges will be used
to dispatch to protocols.

central include files (like <common/common.h>) to specific source files.

purpose never materialized and it causes massive <errno.h> polution.

These reference the actual TCP connection, whereas client.ip and server.ip
may (in a near future) talk about another TCP connection terminating in
a proxy in front of Varnish.

Debian (and Ubuntu) add --disable-maintainer-mode to configure
when building packages, leading to a unrecognised argument warning.

Adding AM_MAINTAINER_MODE disabled keeps the current behavior while
removing the warning.

in the thread pool.

new sess_attr facility

the session workspace to compress struct sess.

Instead one can start gdb on the compiled binary and ask it:

	print sizeof(struct sess)
	$1 = 192

we can reserve the space on the workspace when needed.

scale to parallism.

We don't return 413/414 (yet?) but we have a specific counter for this
now.  Whether this is enough it's something we need to finalise.

into cache_acceptor.c where it belongs.

onto a worker thread (falling back to the current (=acceptor) thread
if the pool is empty.

Use this to save a little bit of stack on common requests.

Rename pool_func_t to task_func_t.

Annotate the task functions with __match_proto__