feat(lua): send "--" literally to Lua "-l" script

Problem:
When "-l" is followed by "--", we stop sending args to the Lua script
and treat "--" in the usual way. This was for flexibility but didn't
have a strong use-case, and has these problems:
- prevents Lua "-l" scripts from handling "--" in their own way.
- complicates the startup logic (must call nlua_init before command_line_scan)

Solution:
Don't treat "--" specially if it follows "-l".

src/nvim/main.h

@@ -24,6 +24,7 @@ typedef struct {
   int n_pre_commands;                   // no. of commands from --cmd
   char *pre_commands[MAX_ARG_CMDS];     // commands from --cmd argument
   char *luaf;                           // Lua script filename from "-l"
+  int lua_arg0;                         // Lua script args start index.
 
   int edit_type;                        // type of editing to do
   char *tagname;                        // tag from -t argument