Treat all HTTP requests under /d/ path is API request

Before:

/d/nonexistent-command returns a file content at
${DOCUMENT_ROOT}/d/nonexistent-command.

After:

/d/nonexistent-command returns '[[-22, 0.0, 0.0], "invalid command
name: nonexistent-command"]'.

Both groonga --protocol http and groonga-httpd are changed.