Remove not used flag CLIENT_CHANGE_USER


Docs/manual.texi:
  Cleanup examples
acconfig.h:
  Fix for HAVE_QUERY_CACHE
configure.in:
  Fix for HAVE_QUERY_CACHE
include/my_global.h:
  Fix for HAVE_QUERY_CACHE
include/mysql_com.h:
  Remove not used flag CLIENT_CHANGE_USER
libmysqld/libmysqld.c:
  Fix new prototypes