Implement T316 to allow RESTART messages to be automatically retransmitted.

Q.931 defines the T316 timer to retransmit RESTART messages if a RESTART
ACKNOWLEDGE message is not received before the timer expires.  Q.931
defaults the time of T316 to 2 minutes with the default number of
consecutive RESTART failures as two.

* To support legacy behavior, the T316 timer is disabled by default.  It
is also disabled because the user cannot configure it to disabled if it is
enabled.

* The N316 count is created to allow the number of RESTART attempts to be
configurable.  Note you will need to recompile Asterisk to be able to
configure N316.

(issue ASTERISK-19608)
(issue AST-815)
(closes issue PRI-133)
Reported by: Mike Boylan
Tested by: rmudgett


git-svn-id: https://origsvn.digium.com/svn/libpri/branches/1.4@2288 2fbb986a-6c06-0410-b554-c9c1f0a7f128

libpri.h

@@ -2179,7 +2179,7 @@ enum PRI_TIMERS_AND_COUNTERS {
 	PRI_TIMER_T310,	/*!< Maximum time between receiving a CALL_PROCEEDING and receiving a ALERT/CONNECT/DISCONNECT/PROGRESS */
 	PRI_TIMER_T313,	/*!< Wait for CONNECT acknowledge, CPE side only */
 	PRI_TIMER_T314,
-	PRI_TIMER_T316,	/*!< Maximum time between transmitting a RESTART and receiving a RESTART ACK */
+	PRI_TIMER_T316,	/*!< Time to wait for a RESTART ACK before retransmitting RESTART. (Timer enabled if greater than zero.) */
 	PRI_TIMER_T317,
 	PRI_TIMER_T318,
 	PRI_TIMER_T319,
@@ -2221,6 +2221,7 @@ enum PRI_TIMERS_AND_COUNTERS {
 	PRI_TIMER_QSIG_CC_T4,	/*!< Path reservation supervision timeout. */
 
 	PRI_TIMER_T312,			/*!< Supervise broadcast SETUP message call reference retention. */
+	PRI_TIMER_N316,			/*!< Number of times to transmit RESTART before giving up if T316 enabled. */
 
 	/* Must be last in the enum list */
 	PRI_MAX_TIMERS