Fix module page restore after full page reload (Ctrl+F5)

[sanchez90]

Fix module page restore after full page reload (Ctrl+F5)

Fixes: #1736

Problem

After a full page reload (Ctrl+F5), authentic-theme sometimes fails to restore
the module page and lands on the dashboard instead.

Root cause

The one-shot "goto" file (written on ?xnavigation=1 redirects to /) is
read and deleted by init_vars() via get_theme_temp_data('goto'). A
background/non-CGI request with an empty REQUEST_URI can consume this
file before the real / request runs init_vars(), so nav_detector() finds
no goto target and falls back to /sysinfo.cgi.

This is a server-side race; it is distinct from (and complementary to) the
existing pragma_no_cache logic, which only addresses the browser-side
heuristic cache of the root page.

Fix

Only consume the goto file on a real page request (when REQUEST_URI is
present). Background requests with an empty REQUEST_URI leave the file
untouched, so the actual / request still finds it.

Testing

Reproduced on Webmin 2.660 / authentic-theme 26.60 with a custom module:

• Before: Ctrl+F5 on /mod/ lands on /sysinfo.cgi (goto consumed by an
  empty-REQUEST_URI background request).
• After: Ctrl+F5 on /mod/ restores /mod/.

Verified that official modules (e.g. /sshd/) continue to restore correctly.

[iliaross]

Hello,

• (goto consumed by an empty-REQUEST_URI background request).

I wonder what would cause REQUEST_URI to be empty. I remember you mentioned that it could happen in the case of a background process. Yet, you never mentioned which background process actually triggers load_theme_library.

I'm not saying the fix is incorrect in principle, and I'm aware of the race in various scenarios. But, I'd prefer to fully understand the reasons why one of the other changes was made.