to work around the lack of a bzero() prototype.

include/m_string.h:
  AIX does have bzero() in its system libraries,
  and the configure phase detects it, including the prototype
  (sets both HAVE_BZERO and HAVE_DECL_BZERO),
  but the declaration is missing when the source is compiled.
  Several attempts all failed.
  This patch takes the brute force approach to always map
  "bzero()" to "memset()" on AIX,
  like is done on platforms where "bzero()" is not found at all.