Co-authored-by: zeertzjq <zeertzjq@outlook.com>

-range=-1 requires the current file to have at least <count> lines, whereas -addr=other doesn't.

-addr=other also sets <count> to -1 by default when it is not specified, though this feature seems undocumented.

Problem:
"set filetype=man" assumes the user wants :Man features, this does extra
stuff like renaming the buffer as "man://".

Solution:
- old entrypoint was ":set filetype=man", but this is too presumptuous #15487
- make the entrypoints more explicit:
  1. when the ":Man" command is run
  2. when a "man://" buffer is opened
- remove the tricky b:man_sect checks in ftplugin/man.vim and syntax/man.vim
- MANPAGER is supported via ":Man!", as documented.

fixes #15487

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.

mandoc may not handle quoted MANPAGER arguments correctly. E.g. with
    export MANPAGER='nvim -u NORC -c "set ft=man"'
mandoc treats `"set` and `ft=man"'` as separate tokens.

To workaround that, provide :Man! so that MANPAGER can avoid quoting.

closes #9120

- :Man with no arguments opens the manapage for the
  <cWORD> (man buffers) or <cword> (non-man buffers).
- remove now irrelevent comment about -P flag

- fix synopsis highlighting in other locales. Cannot always rely on the first
  line for the section in some locales; instead, use the file path and
  explicitly set b:man_sect to the actual section.
- eliminate separate s:man_args function
- simplify logic: do not reuse buffer content
- introduce b:man_default_sects Fixes #5233
- introduce <Plug>(man_vsplit), <Plug>(man_tab)
- simplify regexps

Closes #5235