GUACAMOLE-622: Implicitly invoke guac_terminal_start() if prompting is required.

src/terminal/terminal.c

@@ -943,6 +943,9 @@ char* guac_terminal_prompt(guac_terminal* terminal, const char* title,
     int pos;
     char in_byte;
 
+    /* Prompting implicitly requires user input */
+    guac_terminal_start(terminal);
+
     /* Print title */
     guac_terminal_printf(terminal, "%s", title);
 

src/terminal/terminal/terminal.h

@@ -652,7 +652,9 @@ void guac_terminal_notify(guac_terminal* terminal);
 /**
  * Reads a single line from this terminal's STDIN, storing the result in a
  * newly-allocated string. Input is retrieved in the same manner as
- * guac_terminal_read_stdin() and the same restrictions apply.
+ * guac_terminal_read_stdin() and the same restrictions apply. As reading input
+ * naturally requires user interaction, this function will implicitly invoke
+ * guac_terminal_start().
  *
  * @param terminal
  *     The terminal to which the provided title should be output, and from