Re-fix how local tag is generated when sending a 481 to an INVITE.

Match our local tag to whatever to-tag was sent in the initial INVITE.
Because the size of the to-tag may not fit in the buffer in the sip_pvt,
it has been changed to a string field.

(closes issue ASTERISK-19892)
reported by Walter Doekes

Review: https://reviewboard.asterisk.org/r/1977
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Merged revisions 369352 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369353 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369367 65c4cc65-6c06-0410-ace0-fbb531ad65f3