Enable a few additional warnings for CI and fix the issues they uncovered

- `-Wmissing-prototypes` and `-Wmissing-variable-declarations` are
  helpful for finding dead code and/or things that should be `static`
  but aren't marked as such.
- `-Wunused-macros` is of similar utility, but for identifying dead macros.
- `-Wunreachable-code` and `-Wunreachable-code-aggressive` do exactly
  what they say: flag unreachable code.

src/pa.c

@@ -220,13 +220,6 @@ pa_dalloc(tsdn_t *tsdn, pa_shard_t *shard, edata_t *edata,
 	pai_dalloc(tsdn, pai, edata, deferred_work_generated);
 }
 
-bool
-pa_shard_retain_grow_limit_get_set(tsdn_t *tsdn, pa_shard_t *shard,
-    size_t *old_limit, size_t *new_limit) {
-	return pac_retain_grow_limit_get_set(tsdn, &shard->pac, old_limit,
-	    new_limit);
-}
-
 bool
 pa_decay_ms_set(tsdn_t *tsdn, pa_shard_t *shard, extent_state_t state,
     ssize_t decay_ms, pac_purge_eagerness_t eagerness) {