Revert xmalloc/xrealloc replacements.

* This reverts r3646 and r3645.
* xrealloc/xmalloc are there for a reason:
** Every call to these functions does not check the return value of
   the memory allocation, hence error checking must be performed
   in the wrapper.
** This was historically done in the macros defined in dmemory.h,
   but redefining any identifier from the standard library to
   something which behaves differently is confusing.