AI 144595: Fix the AVD configuration code to support "sdcard.path" in config.ini...

AI 144595: Fix the AVD configuration code to support "sdcard.path" in config.ini to indicate an explicit SD Card image file (instead of using the one in the content directory)
  Note that this also fix a bug where the SD Card image was not properly locked in the previous implementation.
  Allow the http-proxy support code to actually manage to receive chunked encoding data, instead of complaining needlessly.
  Introduce a new CharBuffer object that is used indirectly by "-radio <hostdevice>" and "-gps <hostdevice>" options
  Add new documentation for QEMUD and CharDriverState objects
  Update the Audio documentation with ASCII graphics (because I'm an artist too)
  BUG=1750126,1731936

Automated import of CL 144595