Make form in request_filesystem_credentials() in line with WP admin form styling / HTML structure / security

[dashaluna]

Currently, the Connection Information form output by request_filesystem_credentials() looks very sad and doesn't reflect the admin form HTML structure and style.

This is how it looks: Screen Shot 2016-07-27 at 13.41.52.png​

It would be great if it can be standardised to look like WP admin settings UI, i.e. Screen Shot 2016-07-27 at 13.41.23.png​

While at it, it would be great to secure the form code with escape family functions (esc_attr, esc_html and so on).

Connection Information admin form

General Settings admin form

Related / possible duplicate of: #36973

Oops, sorry I got confused how to add images in the description and now I can't edit it :( Added images as attachments.

On the accessibility front, these fields should probably be changed to stop using implicit labels, but I don't see any other a11y issues from reviewing the code.