The info removal code was overcomplicated, and error-prone (references being wrongly decreased).  Fixed.

  1. Added code to the memory leak detecting code to give the user the
     possibility to add information, thereby forming a traceback.

  2. Make the memory leak detecting code multithread-safe.

The idea is that we're actually dealing with two separate critical
sections, one containing the hash tables with the information, the
other containing the current memory checking mode.  Those should not
be handled with the same lock, especially since their handling overlap.
Hence, the added second lock.

and make it the default for some debugging configurations.

This is much more helpful than the counter when doing tests with the library
interactively.

a pain to read when using 4-space tabs.

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