scripts/mdlinkcheck: markdown link checker

This script parses all markdown files in the repository, extracts all
links and verifies that they work.

It makes sure to only check the URLs once, even if used in multiple
links. There is a whitelist for URLs we deem unnecessary to check.

It uses curl to do the checks.

As a bonus, this makes it easy to run this check locally.

Closes #15742

.github/workflows/checkdocs.yml

@@ -85,22 +85,14 @@ jobs:
   #      - name: check special prose
   #        run: proselint docs/internals/CHECKSRC.md docs/libcurl/curl_mprintf.md docs/libcurl/opts/CURLOPT_INTERFACE.md docs/cmdline-opts/interface.md
 
-  # Docs: https://github.com/UmbrellaDocs/action-linkspector
   linkcheck:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
         name: checkout
 
-      - name: trim the cmdline docs markdown files
-        run: find docs/cmdline-opts -name "*.md" ! -name "_*" ! -name MANPAGE.md -print0 | xargs -0 -n1 .github/scripts/cleancmd.pl
-
-      - name: Run linkspector
-        uses: umbrelladocs/action-linkspector@fc382e19892aca958e189954912fe379a8df270c # v1
-        with:
-          github_token: ${{ secrets.github_token }}
-          reporter: github-pr-review
-          fail_on_error: true
+      - name: Run mdlinkcheck
+        run: ./scripts/mdlinkcheck
 
   spellcheck:
     runs-on: ubuntu-latest