Add --with-mangling.

Add the --with-mangling configure option, which can be used to specify
name mangling on a per public symbol basis that takes precedence over
--with-jemalloc-prefix.

Expose the memalign() and valloc() overrides even if
--with-jemalloc-prefix is specified.  This change does no real harm, and
simplifies the code.
diff --git a/test/thread_arena.c b/test/thread_arena.c
index ef8d681..2922d1b 100644
--- a/test/thread_arena.c
+++ b/test/thread_arena.c
@@ -18,22 +18,22 @@
 	size_t size;
 	int err;
 
-	p = JEMALLOC_P(malloc)(1);
+	p = malloc(1);
 	if (p == NULL) {
 		fprintf(stderr, "%s(): Error in malloc()\n", __func__);
 		return (void *)1;
 	}
 
 	size = sizeof(arena_ind);
-	if ((err = JEMALLOC_P(mallctl)("thread.arena", &arena_ind, &size,
-	    &main_arena_ind, sizeof(main_arena_ind)))) {
+	if ((err = mallctl("thread.arena", &arena_ind, &size, &main_arena_ind,
+	    sizeof(main_arena_ind)))) {
 		fprintf(stderr, "%s(): Error in mallctl(): %s\n", __func__,
 		    strerror(err));
 		return (void *)1;
 	}
 
 	size = sizeof(arena_ind);
-	if ((err = JEMALLOC_P(mallctl)("thread.arena", &arena_ind, &size, NULL,
+	if ((err = mallctl("thread.arena", &arena_ind, &size, NULL,
 	    0))) {
 		fprintf(stderr, "%s(): Error in mallctl(): %s\n", __func__,
 		    strerror(err));
@@ -57,7 +57,7 @@
 
 	fprintf(stderr, "Test begin\n");
 
-	p = JEMALLOC_P(malloc)(1);
+	p = malloc(1);
 	if (p == NULL) {
 		fprintf(stderr, "%s(): Error in malloc()\n", __func__);
 		ret = 1;
@@ -65,8 +65,7 @@
 	}
 
 	size = sizeof(arena_ind);
-	if ((err = JEMALLOC_P(mallctl)("thread.arena", &arena_ind, &size, NULL,
-	    0))) {
+	if ((err = mallctl("thread.arena", &arena_ind, &size, NULL, 0))) {
 		fprintf(stderr, "%s(): Error in mallctl(): %s\n", __func__,
 		    strerror(err));
 		ret = 1;