munin: support groonga-httpd

The old `[groonga_*_http]` and `[groonga_*_gqtp]` didn't work because
Munin doesn't support `*` in the middle. The new style adds a prefix
to environment variables for groogna-server-http, groonga-httpd and
groonga-server-gqtp:

  * `http_` for groogna-server-http
  * `httpd_` for groonga-httpd
  * `gqtp_` for groonga-server-gqtp

Here are new parameters for groonga-server-http:

  * `env.http_host` (env.host is used as a fallback.)
  * `env.http_post` (10041 is used as a fallback.)
  * `env.http_database_path`
  * `env.http_pid_path`
  * `env.http_query_log_path`

Here are new parameters for groonga-httpd:

  * `env.httpd_host` (env.host is used as a fallback.)
  * `env.httpd_post` (10041 is used as a fallback.)
  * `env.httpd_database_path`
  * `env.httpd_pid_path`
  * `env.httpd_query_log_path`

Here are new parameters for groonga-server-gqtp:

  * `env.gqtp_host` (env.host is used as a fallback.)
  * `env.gqtp_post` (10043 is used as a fallback.)
  * `env.gqtp_database_path`
  * `env.gqtp_pid_path`
  * `env.gqtp_query_log_path`

Here are other obsolete parameters:

  * `env.pid_file`: Use `env.pid_path` instead.
  * `env.path`: Use `env.database_path` instead.
  * `env.log_path`: Use `env.query_log_path` instead.

They will be removed when Groonga 5.0.0.

Reported by Naoya Murakami. Thanks!!!