- Made sure some changed behavior is documented in CHANGES.

 - Moved the handling of compile-time defaults from crypto.h to
   mem_dbg.c, since it doesn't make sense for the library users to try
   to affect this without recompiling libcrypto.
 - Made sure V_CRYPTO_MDEBUG_TIME and V_CRYPTO_MDEBUG_THREAD had clear
   and constant definitions.
 - Aesthetic correction.