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);
diff --git a/configure.ac b/configure.ac
index 02842b6..724bc1a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -417,7 +417,7 @@
 AC_PATH_PROG([LD], [ld], [false], [$PATH])
 AC_PATH_PROG([AUTOCONF], [autoconf], [false], [$PATH])
 
-public_syms="malloc_conf malloc_message malloc calloc posix_memalign aligned_alloc realloc free malloc_usable_size malloc_stats_print mallctl mallctlnametomib mallctlbymib"
+public_syms="malloc_conf malloc_message malloc calloc posix_memalign aligned_alloc realloc free mallocx rallocx xallocx sallocx dallocx nallocx mallctl mallctlnametomib mallctlbymib malloc_stats_print malloc_usable_size"
 
 dnl Check for allocator-related functions that should be wrapped.
 AC_CHECK_FUNC([memalign],