main/tcptls: Add support for Perfect Forward Secrecy

This patch enables Perfect Forward Secrecy (PFS) in Asterisk's core TLS API.
Modules that wish to enable PFS should consider the following:

- Ephemeral ECDH (ECDHE) is enabled by default. To disable it, do not
  specify a ECDHE cipher suite in a module's configuration, for example:
  tlscipher=AES128-SHA:DES-CBC3-SHA

- Ephemeral DH (DHE) is disabled by default. To enable it, add DH parameters
  into the private key file, i.e., tlsprivatekey. For an example, see the
  default dh2048.pem at
http://www.opensource.apple.com/source/OpenSSL098/OpenSSL098-35.1/src/apps/dh2048.pem?txt

- Because clients expect the server to prefer PFS, and because OpenSSL sorts
  its cipher suites by bit strength, (see "openssl ciphers -v DEFAULT")
  consider re-ordering your cipher suites in the conf file. For example:
tlscipher=AES128+kEECDH:AES128+kEDH:3DES+kEDH:AES128-SHA:DES-CBC3-SHA:-ADH:-AECDH
  will use PFS when offered by the client. Clients which do not offer PFS
  fall-back to AES-128 (or even 3DES as recommend by RFC 3261).

Review: https://reviewboard.asterisk.org/r/3647/

ASTERISK-23905 #close
Reported by: Alexander Traud
patches:
  tlsPFS_for_HEAD.patch uploaded by Alexander Traud (License 6520)
  tlsPFS.patch uploaded by Alexander Traud (License 6520)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@417803 65c4cc65-6c06-0410-ace0-fbb531ad65f3