api: Allow injection of user-specified malloc/free functions

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

ares_library_init.3

@@ -22,6 +22,7 @@ ares_library_init \- c-ares library initialization
 .B #include <ares.h>
 .PP
 .B int ares_library_init(int \fIflags\fP)
+.B int ares_library_init_mem(int \fIflags\fP, void *(*\fIamalloc\fP)(size_t), void (*\fIafree\fP)(void *))
 .PP
 .B cc file.c -lcares
 .fi
@@ -51,6 +52,12 @@ ORing the values together. In normal operation you should specify
 \fIARES_LIB_INIT_ALL\fP. Don't use any other value unless you are
 familiar with it and trying to control some internal c-ares feature.
 .PP
+The
+.B ares_library_init_mem
+function allows the caller to provide memory management
+functions that the c-ares library will be use instead of \fImalloc(3)\fP and
+\fIfree(3)\fP.
+.PP
 .B This function is not thread safe.
 You have to call it once the program has started, but this call must be done
 before the program starts any other thread. This is required to avoid