SSL/secure socket abstraction and SIP TLS transport rewrite

[nanang]

Objective:

To create a portable SSL/secure socket abstraction in PJLIB.

Specifications:

PJLIB:

• asynchronous I/O API similar to PJLIB's ​Active Socket API
• supports various backends such as OpenSSL, Symbian's CSecureSocket, and Windows SSPI.
• (limited) certificate management API, for backends that supports it.

PJSIP:

• rewrite of PJSIP's SIP SSL transport to use the new PJLIB secure socket.

Implementation:

This initial implementation supports:

• OpenSSL
• CSecureSocket on Symbian

Limitations:

• CSecureSocket does not support server socket (SDK limitation).
• Servername verification may not work.
• CSecureSocket cannot specify certificate when required by server (SDK limitation?)

[nanang]

In r2913, initial version of TLS transport for Symbian, includes:

• Secure socket, generic abstraction and Symbian implementation (using CSecureSocket).
• Initial rewrite of SIP TLS transport.
• Updated symbian_ua.mmp to support SIP transport TLS (experimental).

[nanang]

In r2950:

• Added SSL socket abstraction with OpenSSL backend (source only, i.e: hasn't been integrated to build settings).
• Updated cipher data type and added cipher constants (Symbian SSL socket has also been updated).
• Updated SIP TLS transport to allow setting certificate/credential (via file).