Add parking extension for non-default parking lots.

This is a new feature that allows for parking to custom parking lots to be
accessed directly, rather than with channel variables or by changing the
default parking lot. The extension is set with the parkext option just as the
default parking lot is done. Also, the manager action has been updated to
optionally allow a specified parking lot.

(closes issue #14882)
Reported by: vmikhnevych
Patches: 
      patch_14882.txt uploaded by mnick (license 874)
      modified by me

Review: https://reviewboard.asterisk.org/r/884/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@286931 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/sip/include/sip.h

@@ -768,6 +768,7 @@ struct sip_dual {
 	struct ast_channel *chan2;   /*!< Second channel involved */
 	struct sip_request req;      /*!< Request that caused the transfer (REFER) */
 	int seqno;                   /*!< Sequence number */
+	const char *parkexten;
 };
 
 /*! \brief Parameters to the transmit_invite function */