Treat 0 as no limit for the remaining limits

set_keep_alive_max_count(0) tripped an assert, a negative WebSocket ping
interval sent pings in a tight loop, and the form length, multipart file
count and range count macros rejected every request at 0. Make 0 mean
no limit for those, and a ping interval of 0 or less turn pings off. The
URI, header and line bounds cannot be lifted, so a static_assert keeps
them positive.

docs-src/pages/en/cookbook/s20-keep-alive.md

@@ -10,7 +10,7 @@ status: "draft"
 
 | API | Default | Meaning |
 | --- | --- | --- |
-| `set_keep_alive_max_count` | 100 | Max requests served over a single connection |
+| `set_keep_alive_max_count` | 100 | Max requests served over a single connection (0 for no limit) |
 | `set_keep_alive_timeout` | 5s | How long an idle connection is kept before closing |
 
 ## Basic usage

docs-src/pages/ja/cookbook/s20-keep-alive.md

@@ -10,7 +10,7 @@ status: "draft"
 
 | API | デフォルト | 意味 |
 | --- | --- | --- |
-| `set_keep_alive_max_count` | 100 | 1本の接続で受け付けるリクエストの最大数 |
+| `set_keep_alive_max_count` | 100 | 1本の接続で受け付けるリクエストの最大数（0 で無制限） |
 | `set_keep_alive_timeout` | 5秒 | アイドル状態の接続を閉じるまでの秒数 |
 
 ## 基本の使い方