drop io.lines()

I'd already dropped the variant without a filename. But even the variant
with a filename is very easy to use in a way that is confusing to use in
the presence of sandboxing:

* call io.lines()
* Sandboxing saves an error, io.lines() returns nil
* Caller (usually a loop) raises an error.
* We show the error and not the sandboxing failure.
* Worse, there's no way to adjust permissions from within Teliva,
  because we can't ever get to that menu while there's an error.

Best solution I can come up with: encourage a separate step for
translating filename to file handle. That way it's more obvious that we
need to check for errors.

life.tlv

@@ -160,22 +160,13 @@
     >    while (os.clock() < sec) do
     >    end
     >end
-- __teliva_timestamp: original
-  file_exists:
-    >function file_exists(filename)
-    >  local f = io.open(filename, "r")
-    >  if f ~= nil then
-    >    io.close(f)
-    >    return true
-    >  else
-    >    return false
-    >  end
-    >end
 - __teliva_timestamp: original
   load_file:
     >function load_file(window, filename)
+    >  local infile = io.open(filename, 'r')
+    >  if infile == nil then return end
     >  local line_index = lines
-    >  for line in io.lines(filename) do
+    >  for line in infile:lines() do
     >    if line:sub(1,1) ~= '!' then  -- comment; plaintext files can't have whitespace before comments
     >      local col_index = cols
     >      for c in line:gmatch(".") do
@@ -286,7 +277,7 @@
     >    grid[8][5] = 1
     >    grid[7][4] = 1
     >    grid[6][3] = 1
-    >  elseif file_exists(arg[1]) then
+    >  else
     >    -- Load a file in the standard "plaintext" format: https://www.conwaylife.com/wiki/Plaintext
     >    --
     >    -- Each pattern page at https://www.conwaylife.com/wiki provides its

src/liolib.c

@@ -38,12 +38,6 @@ static int pushresult (lua_State *L, int i, const char *filename) {
 }
 
 
-static void fileerror (lua_State *L, int arg, const char *filename) {
-  lua_pushfstring(L, "%s: %s", filename, strerror(errno));
-  luaL_argerror(L, arg, lua_tostring(L, -1));
-}
-
-
 #define tofilep(L)	((FILE **)luaL_checkudata(L, 1, LUA_FILEHANDLE))
 
 
@@ -171,26 +165,6 @@ static int f_lines (lua_State *L) {
 }
 
 
-static int io_lines (lua_State *L) {
-  const char *filename = luaL_checkstring(L, 1);
-  FILE **pf = newfile(L);
-  static char buffer[1024] = {0};
-  memset(buffer, '\0', 1024);
-  snprintf(buffer, 1020, "io.lines(\"%s\")", filename);
-  append_to_audit_log(L, buffer);
-  if (file_operation_permitted(caller(L), filename, "r"))
-    *pf = fopen(filename, "r");
-  else {
-    snprintf(iolib_errbuf, 1024, "app tried to open file '%s'; adjust its permissions (ctrl-p) if that is expected", filename);
-    Previous_message = iolib_errbuf;
-  }
-  if (*pf == NULL)
-    fileerror(L, 1, filename);
-  aux_lines(L, lua_gettop(L), 1);
-  return 1;
-}
-
-
 /*
 ** {======================================================
 ** READ
@@ -393,7 +367,7 @@ static const luaL_Reg iolib[] = {
   {"close", io_close},
   /* no 'flush' since Teliva is ncurses-based */
   /* no 'input' since Teliva is ncurses-based */
-  {"lines", io_lines},
+  /* no 'io.lines'; it can confusingly fail without showing sandboxing errors */
   {"open", io_open},
   /* no 'output' since Teliva is ncurses-based */
   /* no 'popen' without sandboxing it */