Add a new ares_library_init_mem() initialization function for the
library which allows the library user to specify their own malloc,
realloc & free equivalents for use library-wide.
Store these function pointers in library-wide global variables,
defaulting to libc's malloc(), realloc() and free().
Change all calls to malloc, realloc and free to use the function pointer
instead. Also ensure that ares_strdup() is always available
(even if the local environment includes strdup(3)), and change the
library code to always use it.
Convert calls to calloc() to use ares_malloc() + memset
This commit updates the documentation of ares_library_init() and
ares_library_cleanup() with regard to the newly introduced reference
counting of initializations and deinitializations.
Carsten Larsen
David Drysdale
Alexander Klauer
Daniel Stenberg
Yang Tse