MemoryTracking: adjust curl_domalloc() and curl_dofree() memory filling

Until 2011-08-17 libcurl's Memory Tracking feature also performed
automatic malloc and free filling operations using 0xA5 and 0x13
values. Our own preinitialization of dynamically allocated memory
might be useful when not using third party memory debuggers, but
on the other hand this would fool memory debuggers into thinking
that all dynamically allocated memory is properly initialized.

As a default setting, libcurl's Memory Tracking feature no longer
performs preinitialization of dynamically allocated memory on its
own. If you know what you are doing, and really want to retain old
behavior, you can achieve this compiling with preprocessor symbols
CURL_MT_MALLOC_FILL and CURL_MT_FREE_FILL defined with appropriate
values.