ftplugin: (z)sh: don't assume MANPAGER makes sense

Say you use Vim and set MANPAGER='vim -M +MANPAGER --not-a-term -'; then
:{Zs,S}hKeywordPrg (or K) will crap out and spew terminal garbage into
less when bash's "help" fails. This was introduced by 2f25e40
(runtime: configure keywordpg for some file types (#5566), 2023-08-23)
and may be present in other files touched by that commit.

Make the "man" invocation sensible by unsetting MANPAGER in the
environment.

Note that changing MANPAGER for `:terminal` is not needed; Vim within
Vim is perfectly fine.

runtime/ftplugin/sh.vim

@@ -46,11 +46,11 @@ endif
 
 if get(b:, "is_bash", 0)
  if !has("gui_running") && executable("less")
- command! -buffer -nargs=1 ShKeywordPrg silent exe '!bash -c "{ help "<args>" 2>/dev/null || man "<args>"; } | LESS= less"' | redraw!
+ command! -buffer -nargs=1 ShKeywordPrg silent exe '!bash -c "{ help "<args>" 2>/dev/null || MANPAGER= man "<args>"; } | LESS= less"' | redraw!
  elseif has("terminal")
  command! -buffer -nargs=1 ShKeywordPrg silent exe ':term bash -c "help "<args>" 2>/dev/null || man "<args>""'
  else
- command! -buffer -nargs=1 ShKeywordPrg echo system('bash -c "help <args>" 2>/dev/null || man "<args>"')
+ command! -buffer -nargs=1 ShKeywordPrg echo system('bash -c "help <args>" 2>/dev/null || MANPAGER= man "<args>"')
  endif
  setlocal keywordprg=:ShKeywordPrg
  let b:undo_ftplugin ..= " | setl kp< | sil! delc -buffer ShKeywordPrg"

runtime/ftplugin/zsh.vim

@@ -24,7 +24,7 @@ if executable('zsh') && &shell !~# '/\%(nologin\|false\)$'
  elseif has('terminal')
  command! -buffer -nargs=1 ZshKeywordPrg silent exe ':term zsh -c "autoload -Uz run-help; run-help <args>"'
  else
- command! -buffer -nargs=1 ZshKeywordPrg echo system('zsh -c "autoload -Uz run-help; run-help <args> 2>/dev/null"')
+ command! -buffer -nargs=1 ZshKeywordPrg echo system('MANPAGER= zsh -c "autoload -Uz run-help; run-help <args> 2>/dev/null"')
  endif
  if !exists('current_compiler')
  compiler zsh