Fix severe SIGHUP reload slowdown caused by store_name quadratic search

When reloading large hosts files (e.g. 1.7M addn-hosts entries) on SIGHUP,
free_names() previously only reset block->last = 0 without unlinking or
moving blocks to a spare pool. As a result, all existing blocks remained
in hostblocks. When reloading, the head block was filled first and stayed
at the head while full, forcing store_name() to linearly traverse all
previously filled blocks for every subsequent name insertion. This resulted
in O(N^2) complexity, taking ~10 minutes for 1.7M records.

Fix this by introducing nameblock_spare (matching config_spare / big_free
spare pool semantics), moving freed blocks to the spare pool in free_names(),
and restricting store_name() to only check the active head block before
popping from nameblock_spare (or mallocing). This restores O(1) per-entry
insertion and reduces reload time from 10 minutes to ~3 seconds.

src/cache.c

@@ -217,6 +217,8 @@ static void free_config_crec(struct crec *p)
   config_spare  = p;
 }
 
+static struct nameblock *nameblock_spare = NULL;
+
 static char *store_name(unsigned int namelen, unsigned int index)
 {
   struct nameblock *block;
@@ -225,21 +227,28 @@ static char *store_name(unsigned int namelen, unsigned int index)
   if (namelen > NAMEBLOCK_CHARS)
     return NULL;
   
-  for (block = hostblocks; block; block = block->next)
-    if (block->index == index && NAMEBLOCK_CHARS - block->last >= namelen)
-      break;
+  if (hostblocks && hostblocks->index == index && NAMEBLOCK_CHARS - hostblocks->last >= namelen)
+    {
+      ret = &hostblocks->data[hostblocks->last];
+      hostblocks->last += namelen;
+      return ret;
+    }
 
-  if (!block && ((block = whine_malloc(sizeof(struct nameblock)))))
+  if (nameblock_spare)
+    {
+      block = nameblock_spare;
+      nameblock_spare = nameblock_spare->next;
+    }
+  else
+    block = whine_malloc(sizeof(struct nameblock));
+
+  if (block)
     {
       block->next = hostblocks;
       block->index = index;
+      block->last = namelen;
       hostblocks = block;
-    }
-  
-  if (block && NAMEBLOCK_CHARS - block->last >= namelen)
-    {
-      ret = &block->data[block->last];
-      block->last += namelen;
+      ret = &block->data[0];
     }
 
   return ret;
@@ -247,11 +256,20 @@ static char *store_name(unsigned int namelen, unsigned int index)
 
 static void free_names(unsigned int index)
 {
-  struct nameblock *block;
+  struct nameblock *block, *tmp, **up;
 
-  for (block = hostblocks; block; block = block->next)
-    if (index == UID_NONE || block->index == index)
-      block->last = 0;
+  for (up = &hostblocks, block = hostblocks; block; block = tmp)
+    {
+      tmp = block->next;
+      if (index == UID_NONE || block->index == index)
+	{
+	  *up = tmp;
+	  block->next = nameblock_spare;
+	  nameblock_spare = block;
+	}
+      else
+	up = &block->next;
+    }
 }
 
 /* In most cases, we create the hash table once here by calling this with (hash_table == NULL)