Merge team/russell/sla_rewrite

This is a completely new implementation of the SLA functionality introduced in
Asterisk 1.4.  It is now functional and ready for testing.  However, I will be
adding some additional features over the next week, as well.

For information on how to set this up, see configs/sla.conf.sample 
and doc/sla.txt.

In addition to the changes in app_meetme.c for the SLA implementation itself,
this merge brings in various other changes:

chan_sip:
 - Add the ability to indicate HOLD state in NOTIFY messages.
 - Queue HOLD and UNHOLD control frames even if the channel is not bridged to
   another channel.

linkedlists.h:
 - Add support for rwlock based linked lists.

dial.c:
 - Add the ability to run ast_dial_start() without a reference channel to
   inherit information from.


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@53810 65c4cc65-6c06-0410-ace0-fbb531ad65f3

doc/sla.txt

@@ -0,0 +1,93 @@
+-------------------------------------------------------------
+--- Shared Line Appearances ---------------------------------
+-------------------------------------------------------------
+
+-------------------------------------------------------------
+INTRODUCTION
+
+The "SLA" functionality in Asterisk is intended to allow a setup that emulates
+a simple key system.  It uses the various abstraction layers already built into
+Asterisk to emulate key system functionality across various devices, including
+IP channels.
+-------------------------------------------------------------
+
+-------------------------------------------------------------
+DIALPLAN CONFIGURATION
+
+The SLA implementation can automatically generate the dialplan necessary for
+basic operation if the "autocontext" option is set for trunks and stations in
+sla.conf.  However, for reference, here is an automatically generated dialplan
+to help with custom building of the dialplan to include other features, such as
+voicemail:
+
+[line1]
+exten => s,1,SLATrunk(line1)
+
+[line2]
+exten => s,2,SLATRUNK(line2)
+
+[sla_stations]
+exten => station1,1,SLAStation(station1)
+exten => station1_line1,hint,SLA:station1_line1
+exten => station1_line1,1,SLAStation(station1_line1)
+exten => station1_line2,hint,SLA:station1_line2
+exten => station1_line2,1,SLAStation(station1_line2)
+
+exten => station2,1,SLAStation(station2)
+exten => station2_line1,hint,SLA:station2_line1
+exten => station2_line1,1,SLAStation(station2_line1)
+exten => station2_line2,hint,SLA:station2_line2
+exten => station2_line2,1,SLAStation(station2_line2)
+
+exten => station3,1,SLAStation(station3)
+exten => station3_line1,hint,SLA:station3_line1
+exten => station3_line1,1,SLAStation(station3_line1)
+exten => station3_line2,hint,SLA:station3_line2
+exten => station3_line2,1,SLAStation(station3_line2)
+-------------------------------------------------------------
+
+-------------------------------------------------------------
+TRUNKS
+
+For the trunk side of SLA, the only channels that are currently supported are
+Zap channels.  Support for IP trunks is planned, but not yet implemented.
+
+Be sure to configure the trunk's context to be the same one that is set for the
+"autocontext" option in sla.conf if automatic dialplan configuration is used.
+
+If the dialplan is being built manually, ensure that calls coming in on a trunk
+execute the SLATrunk() application with an argument of the trunk name.
+-------------------------------------------------------------
+
+
+-------------------------------------------------------------
+STATIONS
+
+Currently, the only channel driver that has all of the features necessary to
+support an SLA environment is chan_sip.  Here are some hints on configuring
+a SIP phone for use with SLA:
+
+1) Add the SIP channel as a [station] in sla.conf.
+
+2) Configure the phone in sip.conf.  If automatic dialplan configuration was
+   used by enabling the "autocontext" option in sla.conf, then this entry in
+   sip.conf should have the same context setting.
+
+3) On the phone itself, there are various things that must be configured to
+   make everything work correctly:
+
+   Let's say this phone is called "station1" in sla.conf, and it uses trunks
+   named "line1" and line2".
+
+   a) Two line buttons must be configured to subscribe to the state of the
+      following extensions:
+        - station1_line1
+        - station1_line2
+
+   b) The line appearance buttons should be configured to dial the extensions
+      that they are subscribed to when they are pressed.
+
+   c) If you would like the phone to automatically connect to a trunk when it
+      is taken off hook, then the phone should be automatically configured to
+      dial "station1" when it is taken off hook.
+-------------------------------------------------------------