Disable DEC Special Graphics at exit if vt100

If a vt100 terminal receives the Shift In character '\016' it will
enable the 7 bit DEC Special Graphics character set used for line drawing.

For most users this can happen due to line noise from the tty device and
will likely mess up your terminal even after tio exits.

To better handle this we want to make sure that tio disables this mode
by sending the Shift Out character '\017' at exit.

This mechanism will only activate if environment variable TERM assumes
value "vt100".

src/options.c

@@ -118,6 +118,7 @@ struct option_t option =
     .exclude_drivers = NULL,
     .exclude_tids = NULL,
     .hex_n_value = 0,
+    .vt100 = false,
 };
 
 void print_help(char *argv[])
@@ -487,6 +488,13 @@ void options_parse(int argc, char *argv[])
         option.color = -1;
     }
 
+    // Check for vt100 terminal
+    char *term = getenv("TERM");
+    if ((term != NULL) && (!strcmp(term, "vt100")))
+    {
+        option.vt100 = true;
+    }
+
     while (1)
     {
         static struct option long_options[] =

src/options.h

@@ -95,6 +95,7 @@ struct option_t
     const char *exclude_drivers;
     const char *exclude_tids;
     int hex_n_value;
+    bool vt100;
 };
 
 extern struct option_t option;

src/tty.c

@@ -1067,6 +1067,14 @@ void stdin_configure(void)
 void stdout_restore(void)
 {
     tcsetattr(STDOUT_FILENO, TCSANOW, &stdout_old);
+
+    // If terminal is vt100
+    if (option.vt100)
+    {
+      // Disable DEC Special Graphics character set just in case it was randomly
+      // enabled by noise from serial device.
+      putchar('\017');
+    }
 }
 
 void stdout_configure(void)