Implement the *allocx() API.

Implement the *allocx() API, which is a successor to the *allocm() API.
The *allocx() functions are slightly simpler to use because they have
fewer parameters, they directly return the results of primary interest,
and mallocx()/rallocx() avoid the strict aliasing pitfall that
allocm()/rallocx() share with posix_memalign().  The following code
violates strict aliasing rules:

    foo_t *foo;
    allocm((void **)&foo, NULL, 42, 0);

whereas the following is safe:

    foo_t *foo;
    void *p;
    allocm(&p, NULL, 42, 0);
    foo = (foo_t *)p;

mallocx() does not have this problem:

    foo_t *foo = (foo_t *)mallocx(42, 0);

src/huge.c

@@ -181,7 +181,7 @@ huge_ralloc(void *ptr, size_t oldsize, size_t size, size_t extra,
 #endif
 	{
 		memcpy(ret, ptr, copysize);
-		iqallocx(ptr, try_tcache_dalloc);
+		iqalloct(ptr, try_tcache_dalloc);
 	}
 	return (ret);
 }