dialog: replace timeout_avp with $DLG_timeout

* removed the timeout_avp parameter
* added DLG_timeout pseudovariable for specifying dialog timeout
dialog: add $DLG_timeout pvar documentation
rtpproxy: allow unexisting set

Do not exit if a set is not found during startup, since it may be later on
loaded from database.
nathelper: remove unused nathelper.h header
rtpproxy: remove set_rtp_proxy_set() function

The set_rtp_proxy_set() function has been completely remove. From now on, you
can specify the set directly in the rtpproxy comands. If not specified, the
default set is used.
rtpproxy: allow functions to receive optional parameters
rtpproxy: removed the 'rtpp_sock_pvar' parameter

now all functions return the rtpproxy server used in the 4th optional
parameter
rtpproxy: properly parse the bavp set
rtpproxy: properly store/fetch variables by engage
rtpproxy: updated documentation for the setid and pvar changes
rtpproxy: properly unforce dialog when using engage
dialog: only update timer if the timeout is set on current request