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

Problem:
q in "$MANPAGER mode" does not quit Nvim. This is because
ftplugin/man.vim creates its own mapping:
    nnoremap <silent> <buffer> <nowait> q :lclose<CR><C-W>c
which overrides the one set by the autoload file when using :Man!
("$MANPAGER mode")

Solution:
Set b:pager during "$MANPAGER mode" so that ftplugin/man.vim can set the
mapping correctly.

Fixes #18281
Ref #17791

Helped-by: Gregory Anders <8965202+gpanders@users.noreply.github.com>

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

This also fixes `:Man!`, which wasn't setting 'iskeyword' to contain
parentheses, etc.

Pressing K on manpages with - in their name will now work.

I noticed this the manpages of https://github.com/cli/cli

Not sure why this was added in https://github.com/neovim/neovim/commit/94f4469638590ca5cc724ab6459f2cfc78c621a4

It doesn't seem to do anything and I can't reproduce the linked issue
with this patch so I think it's all working now.

cc @justinmk

Since #11457 this mapping is no longer necessary.
'keywordprg' defaults to :Man in options.lua

man#pop_tag() is also no longer used