dnsdist: Remove jsonp callback, add security HTTP headers

dnsdist: Remove jsonp callback, add security HTTP headers

- Remove the jsonp callback, using simple json data instead (Fixes #3217)
We might need to add CORS if we want to be able to retrieve JSON
data from a webpage not stored on the embedded web server.
- Add several HTTP headers:
 * X-Content-Type-Options: no-sniff to prevent browsers from guessing MIME type
 * X-Frame-Options: deny to prevent clickjacking
 * X-Permitted-Cross-Domain-Policies: none to keep flash from crossing boundaries
 * X-XSS-Protection: 1; mode=block to mitigate XSS
 * Content-Security-Policy: default-src 'self'; img-src *; style-src 'self' 'unsafe-inline',
 a basic CSP policy to restrict which scripts and CSS can be loaded