Add Perfect Forward Secrecy (PFS) support in OpenSSL socket (thanks to Alexander Traud for the patch)

[ming]

Usage (optional):
Append DH parameters into the private key file (privkey_file), for example ​here for ephemeral DH (DHE). Ephemeral ECDH (ECDHE) works
automatically. Without specifying a cipher-suite,
# openssl ciphers -v DEFAULT
is used. Consider reordering or disabling certain suites. Make sure to set
the 'method' parameter to the value 'sslv23' because this disables SSL 2.0
and is the only way to enable TLS 1.2 in pjsip, currently. TLS 1.2 is
required to enable AES-GCM cipher-suites.

Drawback:
For Java clients, go for a 1024bit parameter file, or
disable DHE via 'cipher', or put ECDHE high in priority (of 'cipher').

Speed:
With a mobile phone from 2006 (Nokia E61), DHE/3DES and a 2048bit parameter,
the speed penalty is about 0,5 seconds per connection setup.

[ming]

In 4832:

Fixed #1765: Add PFS support

[riza]

In 4871:

Re #1765:

• Fixed unnecessary white-space error
• Limiting log message to servers
• Adding SSL_OP_SINGLE_ECDH_USE optionally
• OpenSSL could be built without elliptic curve support, or too old