Fix Dynamic Hints Variable Substition - Underscore Problem

When adding a dynamic hint, if an extension contains an underscore no variable
subsitution is being performed.

This patch changes from checking if the extension contains an underscore to
checking if the extension begins with an underscore.

(closes issue ASTERISK-20639)
Reported by: Steven T. Wheeler
Tested by: Steven T. Wheeler, Michael L. Young
Patches:
  asterisk-20639-dynamic-hint-underscore.diff 
                                     uploaded by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/2188/
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Merged revisions 376142 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 376143 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@376144 65c4cc65-6c06-0410-ace0-fbb531ad65f3