Update ptlib to 2.6.1. Lots of changes, no changelog in sight.

Main features on this new major release branch are:

    * Instant Messaging. Using several different ?standards?: RFC4975, MSRP, T.140, SIP-IM etc etc.
    * Much improved support for SIP event handling (SUBSCRIBE/NOTIFY/PUBLISH) using factories so more event packages can be added easily.
    * Support for SIP ?dialog? event package.
    * Support for Shared Line Appearance (aka Multiple Line Appearance, Bridged Line Appearance or Busy Lamp Field)
    * Significant improvements in IPv6 support (thanks Yuri Kiryanov!)
    * Major enhancements to the video rate controller.
    * Removal of the old H.263 ffmpeg plug in and enhancement of the H.263-1998 plug it to take its place.
    * H.224 support (H.323 only).
    * SBC audio codec plug in.
    * G.722 audio codec plug in.
    * CELT audio codec plug in.
    * SWIG support for use in other languages, Java first.
    * Much improved multi-platform sample GUI client, OpenPhone. Now standard test environment replacing simpleOPAL.

(jmcneill)