Hooks to enable custom login without editing core files

[sleepuser]

One of the basic features of wordpress is to avoid having users edit core files.
It is impossible to properly customise wp-login without manually editing the file directly - not good for upgrades!

Need to make login_header function pluggable or add a do_action hook before the <html....> tag
Need to add a login_footer.

In fact what is required was highlighted in #4478 complete with diff file. This was marked as resolved BUT IT WAS NOT! I agree it might be a duplicate of issue #9682 BUT the underlying issue of being able to overwrite the login-header function WAS NOT solved.

(see recent support forum requests #252094 and #333994)

The solution MUST allow a replacement of login header (and even better also a replacement of the </body></html> tags from the switch statement with a login_footer function which could also be overrided - pluggable).

Thanks.

In an ideal world need to re-open #4478 to ensure accurate resolution.

Thanks

[johnbillion]

Duplicate of #17948.