bridge.c: Obey BRIDGE_NOANSWER variable to skip answering channel.

If the BRIDGE_NOANSWER variable is set on a channel, it is not supposed
to answer when another channel bridges to it using Bridge(), and this is
checked when ast_bridge_call* is called. However, another path exists
(bridge_exec -> ast_bridge_add_channel) where this variable was not
checked and channels would be answered. We now check the variable there.

Resolves: #401
Resolves: #1364
(cherry picked from commit a23c467c82)

main/bridge.c

@@ -2578,6 +2578,8 @@ int ast_bridge_add_channel(struct ast_bridge *bridge, struct ast_channel *chan,
 		ast_bridge_unlock(chan_bridge);
 		ast_bridge_unlock(bridge);
 	} else {
+		int noanswer;
+		const char *value;
 		/* Slightly less easy case. We need to yank channel A from
 		 * where he currently is and impart him into our bridge.
 		 */
@@ -2587,9 +2589,17 @@ int ast_bridge_add_channel(struct ast_bridge *bridge, struct ast_channel *chan,
 			ast_bridge_features_destroy(features);
 			return -1;
 		}
-		if (ast_channel_state(yanked_chan) != AST_STATE_UP) {
+
+		ast_channel_lock(chan);
+		value = pbx_builtin_getvar_helper(chan, "BRIDGE_NOANSWER");
+		noanswer = !ast_strlen_zero(value) ? 1 : 0;
+		ast_channel_unlock(chan);
+		if (noanswer) {
+			ast_debug(3, "Skipping answer on bridge target channel %s\n", ast_channel_name(chan));
+		} else if (ast_channel_state(yanked_chan) != AST_STATE_UP) {
 			ast_answer(yanked_chan);
 		}
+
 		ast_channel_ref(yanked_chan);
 		if (ast_bridge_impart(bridge, yanked_chan, NULL, features,
 			AST_BRIDGE_IMPART_CHAN_INDEPENDENT)) {

main/features.c

@@ -172,6 +172,9 @@
 						<para>Additionally, to prevent a bridged channel (the target of the Bridge application)
 						from answering, the <literal>BRIDGE_NOANSWER</literal> variable can be set to inhibit
 						answering.</para>
+						<warning><para>Do not set the <literal>BRIDGE_NOANSWER</literal> variable globally,
+						as it will break normal bridging behavior in many cases. Only use this variable on
+						a per-channel basis when you really know what you are doing!</para></warning>
 					</option>
 					<option name="S(x)">
 						<para>Hang up the call after <replaceable>x</replaceable> seconds *after* the called party has answered the call.</para>