runtime: make command name for &iskeywordprg more unique (#13297)

See https://github.com/vim/vim/pull/13213/commits by @dkearns:
Rename 'keywordprg' user command to ShKeywordPrg as this is just a
leaking implementation detail.

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/readline.vim b/runtime/ftplugin/readline.vim
index a696da2..181d8ac 100644
--- a/runtime/ftplugin/readline.vim
+++ b/runtime/ftplugin/readline.vim
@@ -32,17 +32,17 @@
 
 if has('unix') && executable('less')
   if !has('gui_running')
-    command -buffer -nargs=1 Sman
+    command -buffer -nargs=1 ReadlineKeywordPrg
           \ silent exe '!' . 'LESS= MANPAGER="less --pattern=''^\s+' . <q-args> . '\b'' --hilite-search" man ' . '3 readline' |
           \ redraw!
   elseif has('terminal')
-    command -buffer -nargs=1 Sman
+    command -buffer -nargs=1 ReadlineKeywordPrg
           \ silent exe 'term ' . 'env LESS= MANPAGER="less --pattern=''' . escape('^\s+' . <q-args> . '\b', '\') . ''' --hilite-search" man ' . '3 readline'
   endif
-  if exists(':Sman') == 2
+  if exists(':ReadlineKeywordPrg') == 2
     setlocal iskeyword+=-
-    setlocal keywordprg=:Sman
-    let b:undo_ftplugin .= '| setlocal keywordprg< iskeyword< | sil! delc -buffer Sman'
+    setlocal keywordprg=:ReadlineKeywordPrg
+    let b:undo_ftplugin .= '| setlocal keywordprg< iskeyword< | sil! delc -buffer ReadlineKeywordPrg'
   endif
 endif