Added db_is_table_exists and a few fixes

Added db_is_table_exists.
Fixed a problem when using 'db_rewind' (warning 209: function "db_rewind" should return a value).
Fixed a problem when db_set_asynchronous(DB:db, true/false) always will be DB::SYNCHRONOUS_OFF... now will be:
	* db_set_asynchronous(DB:db, true) -> DB::SYNCHRONOUS_OFF
	* db_set_asynchronous(DB:db, false) -> DB::SYNCHRONOUS_FULL
	!!! My advice about this function ('db_set_asynchronous') seems pointless, removeit
Minor fix db_get_field_float return type from 'return 0' to 'return 0.0'.


I have a question why these functions are like this:
	stock db_rewind(DBResult:dbrResult) {
	stock db_set_row_index(DBResult:dbrResult, iRow) {
	stock db_free_result_hook(DBResult:dbrResult) {
	stock db_set_synchronous(DB:db, DB::e_SYNCHRONOUS_MODE:iValue) {
	stock stmt_bind_value(&DBStatement:stStatement, iParam, DBDataType:iType, {Float, _}:...) {
	stock stmt_skip_row(&DBStatement:stStatement) {
	stock db_dump_table(DB:db, const szTable[], const szFilename[]) {
when they should be
	stock bool:db_rewind(DBResult:dbrResult) {
	stock bool:db_set_row_index(DBResult:dbrResult, iRow) {
	stock bool:db_free_result_hook(DBResult:dbrResult) {
	stock bool:db_set_synchronous(DB:db, DB::e_SYNCHRONOUS_MODE:iValue) {
	stock bool:stmt_bind_value(&DBStatement:stStatement, iParam, DBDataType:iType, {Float, _}:...) {
	stock bool:stmt_skip_row(&DBStatement:stStatement) {
	stock bool:db_dump_table(DB:db, const szTable[], const szFilename[]) {