In InnoDB, the functions mem_alloc() and mem_free() are wrappers for
mem_heap_alloc() and mem_heap_free(). They create an anonymous
InnoDB memory heap for the single user object.

Because the functions are defined inline, their invocation will generate
quite a bit of code.

It is questionable if there is any benefit of using mem_alloc() instead of
ut_malloc(). There clearly is some CPU and memory overhead. The only possible
benefit could be the extra debugging info, which is the subject of
Bug#16924719 69422: small performance impact with heap block debugging info
in release builds

ut_malloc_low(): Remove the always-true parameter and rename to
ut_malloc().  Remove ut_mem_null_ptr. Use a simple assertion also with
the InnoDB memory heaps.

ut_zalloc(): New function, to replace mem_zalloc().

mem_alloc(): Replace with ut_malloc().
mem_free(): Replace with ut_free().

mem_alloc2(): Replace with ut_malloc().
We will no longer determine the true allocated size of the block,
but instead use the requested size only.

os_mem_alloc_large(): Remove an #elif branch that contained non-compiling code.
(The third argument of ut_malloc_low() had been removed a long time ago.)

Do not check if the pointer is NULL before calling ut_free(), because ut_free()
will check that. The caller-side check was needed for mem_free().

rb#3397 approved by Kevin Lewis