2.5.10 patch 3

• All output to stdout/stderr now goes through an internal logging function.
  The log messages can optionally be redirected to a user supplied function
  (by default they are written to stderr).
• Some headers that are internal to the library are no longer installed.
• Restored possibility of compressing the output files (Denis Rosset).
• Remove unused support for dmalloc.
• Convert demo_qs into test_qs TAP test suite.