Find better way for implicit checks in check404

[hannob]

The check404 function does some implicit checks (e.g. webpages with django debugging enabled) by parsing the 404 error page.

This design has the downside that it is not possible to explicitly call those checks (e.g., snallygaster -t djangodebug [host]).

One idea would be to change the code like this:

• There is a function get404page that fetches the 404 page on the first call, and caches it. On subsequent calls it only serves the cached version.
• django_debug and other checks (errorpage_with_php etc.) all go into their own test/function and call this.

This would both allow calling those tests separately, but would not slow them down with multiple fetches if we do a full host scan.

[hannob]

cc @hartwork

[hartwork]

Sounds like the right direction for a start 👍