Fix AMI redirect action with two channels failing to redirect both channels.

The AMI redirect action can fail to redirect two channels that are bridged
together.  There is a race between the AMI thread redirecting the two
channels and the bridge thread noticing that a channel is hungup from the
redirects.

* Made the bridge wait for both channels to be redirected before exiting.

* Made the AMI redirect check that all required headers are present before
proceeding with the redirection.

* Made the AMI redirect require that any supplied ExtraChannel exist
before proceeding.  Previously the code fell back to a single channel
redirect operation.

(closes issue ASTERISK-18975)
Reported by: Ben Klang

(closes issue ASTERISK-19948)
Reported by: Brent Dalgleish
Patches:
      jira_asterisk_19948_v11.patch (license #5621) patch uploaded by rmudgett
Tested by: rmudgett, Thomas Sevestre, Deepak Lohani, Kayode

Review: https://reviewboard.asterisk.org/r/2243/
........

Merged revisions 378356 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 378358 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@378374 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/features.c

@@ -4772,6 +4772,11 @@ before_you_go:
 		silgen = NULL;
 	}
 
+	/* Wait for any dual redirect to complete. */
+	while (ast_test_flag(ast_channel_flags(chan), AST_FLAG_BRIDGE_DUAL_REDIRECT_WAIT)) {
+		sched_yield();
+	}
+
 	if (ast_test_flag(ast_channel_flags(chan), AST_FLAG_BRIDGE_HANGUP_DONT)) {
 		ast_clear_flag(ast_channel_flags(chan), AST_FLAG_BRIDGE_HANGUP_DONT); /* its job is done */
 		if (bridge_cdr) {