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>

test/functional/plugin/man_spec.lua

@@ -2,13 +2,19 @@ local helpers = require('test.functional.helpers')(after_each)
 local Screen = require('test.functional.ui.screen')
 local command, eval, rawfeed = helpers.command, helpers.eval, helpers.rawfeed
 local clear = helpers.clear
+local funcs = helpers.funcs
+local nvim_prog = helpers.nvim_prog
+local matches = helpers.matches
 
 describe(':Man', function()
+  before_each(function()
+    clear()
+  end)
+
   describe('man.lua: highlight_line()', function()
     local screen
 
     before_each(function()
-      clear()
       command('syntax on')
       command('set filetype=man')
       command('syntax off')  -- Ignore syntax groups
@@ -137,4 +143,10 @@ describe(':Man', function()
       ]])
     end)
   end)
+
+  it('q quits in "$MANPAGER mode" (:Man!) #18281', function()
+    -- This will hang if #18281 regresses.
+    local args = {nvim_prog, '--headless', '+autocmd VimLeave * echo "quit works!!"', '+Man!', '+call nvim_input("q")'}
+    matches('quit works!!', funcs.system(args, {'manpage contents'}))
+  end)
 end)