Remove --enable-ivsalloc.

Continue to use ivsalloc() when --enable-debug is specified (and add
assertions to guard against 0 size), but stop providing a documented
explicit semantics-changing band-aid to dodge undefined behavior in
sallocx() and malloc_usable_size().  ivsalloc() remains compiled in,
unlike when #211 restored --enable-ivsalloc, and if
JEMALLOC_FORCE_IVSALLOC is defined during compilation, sallocx() and
malloc_usable_size() will still use ivsalloc().

This partially resolves #580.

include/jemalloc/internal/jemalloc_preamble.h.in

@@ -132,13 +132,6 @@ static const bool config_xmalloc =
     false
 #endif
     ;
-static const bool config_ivsalloc =
-#ifdef JEMALLOC_IVSALLOC
-    true
-#else
-    false
-#endif
-    ;
 static const bool config_cache_oblivious =
 #ifdef JEMALLOC_CACHE_OBLIVIOUS
     true
@@ -164,5 +157,16 @@ static const bool have_percpu_arena =
     false
 #endif
     ;
+/*
+ * Undocumented, and not recommended; the application should take full
+ * responsibility for tracking provenance.
+ */
+static const bool force_ivsalloc =
+#ifdef JEMALLOC_FORCE_IVSALLOC
+    true
+#else
+    false
+#endif
+    ;
 
 #endif /* JEMALLOC_PREAMBLE_H */