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). The call to the
efile driver is blocking. So an unresponsive client could block it for a
very long time. Yaws linked-in driver suffers from the same bug. So, by
default, the use of sendfile (Yaws or erlang) is disable.
Adjust line numbers in known_dialyzer_warnings file