fix(man.vim): q in "$MANPAGER mode" does not quit #18443

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>

runtime/autoload/man.vim

@@ -125,9 +125,7 @@ endfunction
 function! s:set_options(pager) abort
   setlocal noswapfile buftype=nofile bufhidden=hide
   setlocal nomodified readonly nomodifiable
-  if a:pager
-    nnoremap <silent> <buffer> <nowait> q :lclose<CR>:q<CR>
-  endif
+  let b:pager = a:pager
   setlocal filetype=man
 endfunction
 

runtime/ftplugin/man.vim

@@ -24,7 +24,11 @@ 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>
-  nnoremap <silent> <buffer> <nowait> q :lclose<CR><C-W>c
+  if get(b:, 'pager')
+    nnoremap <silent> <buffer> <nowait> q :lclose<CR><C-W>q
+  else
+    nnoremap <silent> <buffer> <nowait> q :lclose<CR><C-W>c
+  endif
 endif
 
 if get(g:, 'ft_man_folding_enable', 0)