ptyfwd: reset tty when exiting

Let's do a "soft" reset of the TTY when a ptyfwd session ends. This is a
good idea, in order to reset changes to the scrolling window that code
inside the session might have made. A "soft" reset will undo this.

While we are at it, make sure to output the ansi sequences for this
*after* terminating any half-written line, as that is still somewhat
contents of the session, even if it's augmented.

src/basic/terminal-util.h

@@ -34,6 +34,8 @@
 
 bool isatty_safe(int fd);
 
+int terminal_reset_ansi_seq(int fd);
+
 typedef enum TerminalResetFlags {
         TERMINAL_RESET_SWITCH_TO_TEXT = 1 << 0,
         TERMINAL_RESET_AVOID_ANSI_SEQ = 1 << 1,