fix(man.vim): filetype=man is too eager #15489

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

runtime/ftplugin/man.vim

@@ -6,14 +6,6 @@ if exists('b:did_ftplugin') || &filetype !=# 'man'
 endif
 let b:did_ftplugin = 1
 
-let s:pager = !exists('b:man_sect')
-
-if s:pager
-  call man#init_pager()
-endif
-
-setlocal noswapfile buftype=nofile bufhidden=hide
-setlocal nomodified readonly nomodifiable
 setlocal noexpandtab tabstop=8 softtabstop=8 shiftwidth=8
 setlocal wrap breakindent linebreak
 
@@ -32,11 +24,7 @@ if !exists('g:no_plugin_maps') && !exists('g:no_man_maps')
   nnoremap <silent> <buffer> k             gk
   nnoremap <silent> <buffer> gO            :call man#show_toc()<CR>
   nnoremap <silent> <buffer> <2-LeftMouse> :Man<CR>
-  if s:pager
-    nnoremap <silent> <buffer> <nowait> q :lclose<CR>:q<CR>
-  else
-    nnoremap <silent> <buffer> <nowait> q :lclose<CR><C-W>c
-  endif
+  nnoremap <silent> <buffer> <nowait> q :lclose<CR><C-W>c
 endif
 
 if get(g:, 'ft_man_folding_enable', 0)