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
........

Merged revisions 407272 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@407273 65c4cc65-6c06-0410-ace0-fbb531ad65f3