Implement malloc_vsnprintf().

Implement malloc_vsnprintf() (a subset of vsnprintf(3)) as well as
several other printing functions based on it, so that formatted printing
can be relied upon without concern for inducing a dependency on floating
point runtime support.  Replace malloc_write() calls with
malloc_*printf() where doing so simplifies the code.

Add name mangling for library-private symbols in the data and BSS
sections.  Adjust CONF_HANDLE_*() macros in malloc_conf_init() to expose
all opt_* variable use to cpp so that proper mangling occurs.

test/posix_memalign.c

@@ -100,7 +100,7 @@ main(void)
 				    alignment, size);
 				if (err) {
 					fprintf(stderr,
-					    "Error for size %zu (0x%zx): %s\n",
+					    "Error for size %zu (%#zx): %s\n",
 					    size, size, strerror(err));
 					exit(1);
 				}