Fix option to enable/disable sendfile support in the configure script

Now it is possible to disable sendfile in yaws passing the option
'--disable-senfile' (or '--enable-sendfile=no') to the configure script.
It is also possible to choose between the erlang implementation (the default)
and the yaws implementation.
Make the #gconf.large_file_chunk_size value be configurable
Add support of many options for tcp/ssl sockets

By adding a "<listen_opts> ... </listen_opts>" block in vhosts
configuration, it is possible to define some options to be set on listen
sockets and, by inheritance, on accepted sockets. Supported options are
(see inet:setopts for details):

  * buffer
  * delay_send
  * linger
  * nodelay
  * priority
  * sndbuf
  * recbuf
  * send_timeout
  * send_timeout_close

send_timeout / send_timeout_close options could be very useful to handle
unresponsive clients and thus avoid some sorts of DoS.
Add the way to configure the version of sendfile method to use

Now, at compile time, we only check if sendfile syscall and file:sendfile/5
are supported. Then, dynamically, we can configure which version to use.
This can be done using the directive large_file_sendfile. Supported values
are:

 * erlang: if supported, use file:sendfile/5
 * yaws: if supported, use the Yaws sendfile linked-in driver
 * disable: do not use any sendfile method, but gen_tcp:send/2

If an unsupported method is configured, we fall back on gen_tcp:send/2.

file:sendfile/5 implementation is buggy (in R15 & R16). When async-threads
are enabled, in efile driver, the tcp socket is set in blocking mode and the
sendfile syscall is executed on an async-thread. So an unresponsive client
could block it for a very long time and therefore block the async-thread.
In this way, all async-threads could be easily blocked.
So, by default, the use of sendfile is disable.
Adjust line numbers in known_dialyzer_warnings file