debugger: nul-terminated string format improvements

* use one argument instead of two
* use width to control maximum length
* add debugger help text

src/emu/debug/debugcmd.cpp

@@ -573,33 +573,35 @@ bool debugger_commands::mini_printf(std::ostream &stream, const std::vector<std:
 					break;
 
 				case 's':
-					if ((param + 1) < params.size())
 					{
-						address_space *space, *tspace;
-						std::string s;
-
-						if (m_console.validate_device_space_parameter(params[param + 0], -1, space) && m_console.validate_number_parameter(params[param + 1], number))
+						address_space *space;
+						if (param < params.size() && m_console.validate_target_address_parameter(params[param++], -1, space, number))
 						{
+							address_space *tspace;
+							std::string s;
+
 							for (u32 address = u32(number), taddress; space->device().memory().translate(space->spacenum(), device_memory_interface::TR_READ, taddress = address, tspace); address++)
 							{
 								u8 const data = tspace->read_byte(taddress);
 
-								if (data)
-									s += data;
-								else
+								if (!data)
 									break;
+
+								s += data;
+
+								if (width == 1)
+									break;
+								else if (width)
+									width--;
 							}
 
-							util::stream_format(stream, "%*s", width, s);
-							param += 2;
+							util::stream_format(stream, "%s", s);
 						}
 						else
+						{
+							m_console.printf("Not enough parameters for format!\n");
 							return false;
-					}
-					else
-					{
-						m_console.printf("Not enough parameters for format!\n");
-						return false;
+						}
 					}
 					break;
 			}

src/emu/debug/debughlp.cpp

@@ -361,6 +361,7 @@ const help_item f_static_help_list[] =
 		"\n"
 		"  %[0][<n>]d -- prints <item> as a decimal value with optional digit count and zero-fill\n"
 		"  %[0][<n>]x -- prints <item> as a hexadecimal value with optional digit count and zero-fill\n"
+		"  %[<n>]s -- prints all or up to <n> non-nul bytes from address and address space given by <item>\n"
 		"\n"
 		"All remaining formatting options are ignored. Use %% together to output a % character. Multiple "
 		"lines can be printed by embedding a \\n in the text.\n"
@@ -383,6 +384,7 @@ const help_item f_static_help_list[] =
 		"\n"
 		"  %[0][<n>]d -- logs <item> as a decimal value with optional digit count and zero-fill\n"
 		"  %[0][<n>]x -- logs <item> as a hexadecimal value with optional digit count and zero-fill\n"
+		"  %[<n>]s -- logs all or up to <n> non-nul bytes from address and address space given by <item>\n"
 		"\n"
 		"All remaining formatting options are ignored. Use %% together to output a % character. Multiple "
 		"lines can be printed by embedding a \\n in the text.\n"