Absolute paths in man page links such as
```
[some link](/foo/bar)
```
Were being interpreted as relative paths when rendered in HTML. This
issue was observed in #20889 and #20041.
The fix is to make sure we don't leave any absolute paths when parsing
links. Instead we check if a directory is absolute (by checking for `/`)
and add `file://` accordingly. This turns the above link into:
```
[some link](file:///foo/bar)
```
Which does get interpreted correctly when rendered as HTML.
- fixes#20889
- fixes#20041
Before this patch, opening the Help application would raise an error.
Now all the pictures in the man pages render correctly.