Fix blind transfer parking issues if the dialed extension is not recognized as a parking extension.

Custom parking extensions may not be coded such that the first and only
extension priority is the Park application.  These custom parking
extensions will not be recognized as parking extensions.  When a call is
blind transferred to an extension that is not recognized as a parking
extension, the normal blind transfer code causes the transferred channel
to start executing dialplan.  Calls that get parked in this manner do not
know the original channel name that parked the call so the original parker
could never be called back if the parked call is not retrieved before the
timeout time.  The parking space is also announced to the call being
parked as a side effect of not knowing the original parking channel.

* Fix handling of BLINDTRANSFER channel variable for call parking.

* Fixed SIP blind transfer using the wrong dialplan context variable to
check for the parking extension.

(closes issue ASTERISK-19322)
Reported by: aragon
Tested by: rmudgett, jparker

Review: https://reviewboard.asterisk.org/r/1730/

JIRA AST-766
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Merged revisions 356521 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 356522 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@356523 65c4cc65-6c06-0410-ace0-fbb531ad65f3