Make eligible global variables static and/or const

For better or worse, Jemalloc has a significant number of global
variables. Making all eligible global variables `static` and/or `const`
at least makes it slightly easier to reason about them, as these
qualifications communicate to the programmer restrictions on their use
without having to `grep` the whole codebase.

src/pa.c

@@ -17,7 +17,7 @@ pa_nactive_sub(pa_shard_t *shard, size_t sub_pages) {
 
 bool
 pa_central_init(pa_central_t *central, base_t *base, bool hpa,
-    hpa_hooks_t *hpa_hooks) {
+    const hpa_hooks_t *hpa_hooks) {
 	bool err;
 	if (hpa) {
 		err = hpa_central_init(&central->hpa, base, hpa_hooks);