tls: partially backport pull request #26415

This is a partial backport of commit f1a3968 ("tls: expose built-in
root certificates") from the master branch. The original commit adds a
new API, this commit just backports the non-visible changes to ease
backporting follow-up commits.

PR-URL: #26415
Backport-PR-URL: #29137
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Ron Korving <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Sam Roberts <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>

Author: bnoordhuis