Added support for memory debuggers (efence and [untested] dmalloc), just

Added support for memory debuggers (efence and [untested] dmalloc), just
enable in the top-level Makefile.

All modules should use xmalloc/xrealloc/xfree (or memory pools) to allocate
their data. These functions automatically check for errors and they also
translate to dmalloc calls if needed.