Add minetest.safe_write_file() to script API

doc/lua_api.txt

@@ -2380,6 +2380,10 @@ Strings that need to be translated can contain several escapes, preceded by `@`.
      * nil: return all entries,
      * true: return only subdirectory names, or
      * false: return only file names.
* `minetest.safe_file_write(path, content)`: returns boolean indicating success
    * Replaces contents of file at path with new contents in a safe (atomic) way.
      Use this instead of below code when writing e.g. database files:
      `local f = io.open(path, "wb"); f:write(content); f:close()`
* `minetest.get_version()`: returns a table containing components of the
   engine version.  Components:
    * `project`: Name of the project, eg, "Minetest"

src/script/lua_api/l_util.cpp

@@ -357,6 +357,23 @@ int ModApiUtil::l_get_dir_list(lua_State *L)
	return 1;
}

// safe_file_write(path, content)
int ModApiUtil::l_safe_file_write(lua_State *L)
{
	NO_MAP_LOCK_REQUIRED;
	const char *path = luaL_checkstring(L, 1);
	size_t size;
	const char *content = luaL_checklstring(L, 2, &size);

	CHECK_SECURE_PATH(L, path, true);

	bool ret = fs::safeWriteToFile(path, std::string(content, size));
	lua_pushboolean(L, ret);

	return 1;
}

// request_insecure_environment()
int ModApiUtil::l_request_insecure_environment(lua_State *L)
{
	NO_MAP_LOCK_REQUIRED;
@@ -476,6 +493,7 @@ void ModApiUtil::Initialize(lua_State *L, int top)

	API_FCT(mkdir);
	API_FCT(get_dir_list);
	API_FCT(safe_file_write);

	API_FCT(request_insecure_environment);

src/script/lua_api/l_util.h

@@ -80,6 +80,9 @@ class ModApiUtil : public ModApiBase
	// get_dir_list(path, is_dir)
	static int l_get_dir_list(lua_State *L);

	// safe_file_write(path, content)
	static int l_safe_file_write(lua_State *L);

	// request_insecure_environment()
	static int l_request_insecure_environment(lua_State *L);