tcptls.c: Made TLS handle a certificate chain file.

Thanks to Guillaume Martres for doing the necessary research to validate
the change.

(closes issue ASTERISK-17727)
Reported by: LN
Patches:
      use_certificate_chain.patch (license #5864) patch uploaded by st
      documente_certificate_chain.patch (license #6576) patch uploaded by Guillaume Martres
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Merged revisions 407272 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 407273 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@407274 f38db490-d61c-443f-a65b-d21fe96a405b