Added support for pressing esc in shell so it escapes the current line and allows you to type a new command

Imported-from: https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@1136

src/shell/shell_misc.cpp

@@ -231,6 +231,16 @@ void DOS_Shell::InputCommand(char * line) {
 				}
 			}
 			break;
+		case 0x1b:   /* ESC */
+			//write a backslash and return to the next line
+			outc('\\');
+			outc('\n');
+			*line = 0;      // reset the line.
+			if (l_completion.size()) l_completion.clear(); //reset the completion list.
+			this->InputCommand(line);	//Get the NEW line.
+			size = 0;       // stop the next loop
+			str_len = 0;    // prevent multiple adds of the same line
+			break;
 		default:
 			if (l_completion.size()) l_completion.clear();
 			line[str_index]=c;