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);

test/integration/xallocx.c

@@ -0,0 +1,59 @@
+#include "test/jemalloc_test.h"
+
+TEST_BEGIN(test_same_size)
+{
+	void *p;
+	size_t sz, tsz;
+
+	p = mallocx(42, 0);
+	assert_ptr_not_null(p, "Unexpected mallocx() error");
+	sz = sallocx(p, 0);
+
+	tsz = xallocx(p, sz, 0, 0);
+	assert_zu_eq(tsz, sz, "Unexpected size change: %zu --> %zu", sz, tsz);
+
+	dallocx(p, 0);
+}
+TEST_END
+
+TEST_BEGIN(test_extra_no_move)
+{
+	void *p;
+	size_t sz, tsz;
+
+	p = mallocx(42, 0);
+	assert_ptr_not_null(p, "Unexpected mallocx() error");
+	sz = sallocx(p, 0);
+
+	tsz = xallocx(p, sz, sz-42, 0);
+	assert_zu_eq(tsz, sz, "Unexpected size change: %zu --> %zu", sz, tsz);
+
+	dallocx(p, 0);
+}
+TEST_END
+
+TEST_BEGIN(test_no_move_fail)
+{
+	void *p;
+	size_t sz, tsz;
+
+	p = mallocx(42, 0);
+	assert_ptr_not_null(p, "Unexpected mallocx() error");
+	sz = sallocx(p, 0);
+
+	tsz = xallocx(p, sz + 5, 0, 0);
+	assert_zu_eq(tsz, sz, "Unexpected size change: %zu --> %zu", sz, tsz);
+
+	dallocx(p, 0);
+}
+TEST_END
+
+int
+main(void)
+{
+
+	return (test(
+	    test_same_size,
+	    test_extra_no_move,
+	    test_no_move_fail));
+}