mod_rewrite: Check UNCList before prefix_stat() probes a UNC path

[arturobernalg]

On Windows, prefix_stat() calls apr_filepath_root() with APR_FILEPATH_TRUENAME on the result of a server-context RewriteRule before any UNC check. For a UNC path this makes APR call GetDriveTypeW() on the \\host\share\ root, which contacts the host. With UNCList unset (all UNC access denied), a RewriteRule that produces a UNC path therefore still triggers a network lookup, and a worker waits for it, before the path is rejected.

This calls ap_stat_check() first, like the file tests of RewriteCond already do since r1927033. ap_stat_check() is a no-op on other platforms. If the check refuses the path, prefix_stat() returns 0, the same result as when the root probe failed, so the final response is unchanged (404) but without the network access. An allowed host still takes the old path.

PR 70210.

The test is Windows only (UNCList does not exist elsewhere), so it is skipped on Unix. I could only check the Unix side locally: mod_rewrite builds and the pytest_suite rewrite tests give the same results as before.

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• This PR does not report or address a security vulnerability
  (see SECURITY.md and
  https://www.apache.org/security/#reporting-a-vulnerability).
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