Patch release for Beta 5 (critical fixes only).

• Segmentation Violation caused by indexing past the end of a subsector plane's light links array when dynamic lights were disabled.

• The Mus and Ext interfaces can be loaded seperately out of a ds plugin.
• As a fallback, if dsSDLMixer is not available, use the Win32 Mus interface.
• The -ds9 command line option reverts back to the DirectSound plugin (with 3D sound support and EAX effects), but MP3/OGG/etc. music playback won't be available in that case.

• Under certain circumstances, Doomsday failed to understand the master server HTTP responses. This was fixed by using an external library (libcurl) for doing the HTTP transfer.