[CI:DOCS] Add CI check for SEE ALSO in man pages

Add new CI check to confirm that links and references
in SEE ALSO sections are properly formatted and that
links are valid (at least in theory: we do no actual
URL fetching to test for 404).

The check is piggybacked into existing xref-helpmsgs-manpages
script. It could conceivably be more elegant to write a
separate tool for this purpose, but I don't wish to duplicate
the logic for finding and reading markdown files.

Script identified various problems, which I fix in this PR:

  . missing '**' (asterisks) around some references, or '**'
    in the wrong place.

  . links pointing to github.com/.../tree/ instead of /blob/
    (github redirects those automatically, but I like
    consistency)

  . a few copy-paste errors, e.g. subgid linking to subuid.

Signed-off-by: Ed Santiago <santiago@redhat.com>

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