runtime: Fix man.vim count handling.

Here I use a negative number to decide whether the count has been
explicitly set. I think it unlikely that negative sections will ever be
created given that negative numbers complicate argument handling:
```
$ man -1 foo
man: invalid option -- '1'
```
and given that there's already precedence for alphanumeric sections like
`3p`, `3x`, `n`, etc.

---

This does work, though:
```
$ man -S -3 baz
```
With `man baz.-3` and `man 'baz(-3)'`, (GNU) man *might* consider `-3`
internally as a section, but in the end reports as if the whole
argument was the name of a topic:
```
$ man 'baz(-3)'
No manual entry for baz(-3)
```

---

Closes #13411.

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