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/man/nvim.1

@@ -122,9 +122,6 @@ modifications.
 .It Fl b
 Binary mode.
 .Ic ":help edit-binary"
-.It Fl l
-Lisp mode.
-Sets the 'lisp' and 'showmatch' options.
 .It Fl A
 Arabic mode.
 Sets the 'arabic' option.
@@ -144,7 +141,7 @@ is specified, append messages to
 instead of printing them.
 .Ic ":help 'verbose'"
 .It Fl D
-Debug mode for VimL (Vim script).
+Vimscript debug mode.
 Started when executing the first command from a script.
 :help debug-mode
 .It Fl n
@@ -268,10 +265,26 @@ but execute
 before processing any vimrc.
 Up to 10 instances of these can be used independently from instances of
 .Fl c .
+.It Fl l Ar script Op Ar args
+Execute Lua
+.Ar script
+with optional
+.Op Ar args
+after processing any preceding Nvim startup arguments.
+All
+.Op Ar args
+are treated as script arguments and are passed literally to Lua, that is,
+.Fl l
+stops processing of Nvim arguments.
+.Ic ":help -l"
 .It Fl S Op Ar session
-Source
+Execute
 .Ar session
-after the first file argument has been read.
+after the first file argument has been read. If
+.Ar session
+filename ends with
+.Pa .lua
+it is executed as Lua instead of Vimscript.
 Equivalent to
 .Cm -c \(dqsource session\(dq .
 .Ar session

src/nvim/lua/executor.c

@@ -323,32 +323,28 @@ static int nlua_thr_api_nvim__get_runtime(lua_State *lstate)
   return 1;
 }
 
-/// Copies all args into the Lua `arg` global.
+/// Copies args starting at `lua_arg0` into the Lua `arg` global.
 ///
 /// Example:
-///     nvim -l foo.lua -- -e "sin=math.sin" script a b
+///     nvim -l foo.lua --arg1 --arg2
 ///
 /// @note `lua` CLI sets _negative_ `arg` indices to the arguments upto "-e".
 ///
 /// @see https://www.lua.org/pil/1.4.html
 /// @see https://github.com/premake/premake-core/blob/1c1304637f4f5e50ba8c57aae8d1d80ec3b7aaf2/src/host/premake.c#L563-L594
 ///
-/// @returns number of args (stops at "--")
-int nlua_set_argv(char **argv, int argc)
+/// @returns number of args
+int nlua_set_argv(char **argv, int argc, int lua_arg0)
 {
   lua_State *const L = global_lstate;
-  lua_newtable(L);
+  lua_newtable(L);  // _G.arg
   int i = 0;
-  for (; i < argc; i++) {
-    if (strequal("--", argv[i])) {
-      i--;
-      break;
-    }
-    lua_pushstring(L, argv[i]);
-    lua_rawseti(L, -2, i + 1);
+  for (; lua_arg0 >= 0 && i + lua_arg0 < argc; i++) {
+    lua_pushstring(L, argv[i + lua_arg0]);
+    lua_rawseti(L, -2, i + 1);  // _G.arg[i+1] = "arg1"
   }
   lua_setglobal(L, "arg");
-  return i + 1;
+  return i;
 }
 
 static void nlua_schedule_event(void **argv)

src/nvim/main.c

@@ -275,14 +275,14 @@ int main(int argc, char **argv)
   // Check if we have an interactive window.
   check_and_set_isatty(&params);
 
-  // TODO: should we try to keep param scan before this?
-  nlua_init();
-  TIME_MSG("init lua interpreter");
-
   // Process the command line arguments.  File names are put in the global
   // argument list "global_alist".
   command_line_scan(&params);
 
+  nlua_init();
+  nlua_set_argv(argv, argc, params.lua_arg0);
+  TIME_MSG("init lua interpreter");
+
   if (embedded_mode) {
     const char *err;
     if (!channel_from_stdio(true, CALLBACK_READER_INIT, &err)) {
@@ -1318,14 +1318,9 @@ static void command_line_scan(mparm_T *parmp)
           }
           parmp->luaf = argv[0];
           argc--;
-          argv++;
-          // Lua args after "-l <file>" (upto "--").
-          int l_argc = nlua_set_argv(argv, argc);
-          assert(l_argc >= 0);
-          argc = argc - l_argc;
-          if (argc > 0) {  // Found "--".
-            argv = argv + l_argc;
-            had_minmin = true;
+          if (argc > 0) {  // Lua args after "-l <file>".
+            parmp->lua_arg0 = parmp->argc - argc;
+            argc = 0;
           }
           break;
 
@@ -1438,6 +1433,7 @@ static void init_params(mparm_T *paramp, int argc, char **argv)
   paramp->server_addr = NULL;
   paramp->remote = 0;
   paramp->luaf = NULL;
+  paramp->lua_arg0 = -1;
 }
 
 /// Initialize global startuptime file if "--startuptime" passed as an argument.

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