CLEANUP: lists/tree-wide: rename some list operations to avoid some confusion

The current "ADD" vs "ADDQ" is confusing because when thinking in terms
of appending at the end of a list, "ADD" naturally comes to mind, but
here it does the opposite, it inserts. Several times already it's been
incorrectly used where ADDQ was expected, the latest of which was a
fortunate accident explained in 6fa922562 ("CLEANUP: stream: explain
why we queue the stream at the head of the server list").

Let's use more explicit (but slightly longer) names now:

   LIST_ADD        ->       LIST_INSERT
   LIST_ADDQ       ->       LIST_APPEND
   LIST_ADDED      ->       LIST_INLIST
   LIST_DEL        ->       LIST_DELETE

The same is true for MT_LISTs, including their "TRY" variant.
LIST_DEL_INIT keeps its short name to encourage to use it instead of the
lazier LIST_DELETE which is often less safe.

The change is large (~674 non-comment entries) but is mechanical enough
to remain safe. No permutation was performed, so any out-of-tree code
can easily map older names to new ones.

The list doc was updated.
diff --git a/src/shctx.c b/src/shctx.c
index f4002a4..1160a1b 100644
--- a/src/shctx.c
+++ b/src/shctx.c
@@ -371,7 +371,7 @@
 		cur_block->len = 0;
 		cur_block->refcount = 0;
 		cur_block->block_count = 1;
-		LIST_ADDQ(&shctx->avail, &cur_block->list);
+		LIST_APPEND(&shctx->avail, &cur_block->list);
 		shctx->nbav++;
 		cur += sizeof(struct shared_block) + blocksize;
 	}