Fix AST_LIST_INSERT_BEFORE_CURRENT() updating the wrong variable.

AST_LIST_INSERT_BEFORE_CURRENT() could not be used twice in an iteration
or before AST_LIST_REMOVE_CURRENT() without corrupting the list.
AST_LIST_INSERT_BEFORE_CURRENT() could also corrupt the list if
AST_LIST_INSERT_BEFORE_CURRENT() or AST_LIST_REMOVE_CURRENT() is used on
the next iteration.

* Fixed cut and paste error using the wrong variable in
AST_LIST_INSERT_BEFORE_CURRENT().

* Added linked list unit tests for AST_LIST_INSERT_BEFORE_CURRENT(),
AST_LIST_APPEND_LIST(), and AST_LIST_INSERT_LIST_AFTER().
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Merged revisions 342661 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 342662 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@342663 65c4cc65-6c06-0410-ace0-fbb531ad65f3