implement status is-breakpoint

This implements `status is-breakpoint` that returns true if the current
shell prompt is displayed in the context of a `breakpoint` command.

This also fixes several bugs. Most notably making `breakpoint` a no-op if
the shell isn't interactive. Also, typing `breakpoint` at an interactive
prompt should be an error rather than creating a new nested debugging
context.

Partial fix for #1310

src/fish.cpp

@@ -255,11 +255,11 @@ static int fish_parse_opt(int argc, char **argv, std::vector<std::string> *cmds)
                 break;
             }
             case 'i': {
-                is_interactive_session = 1;
+                is_interactive_session = true;
                 break;
             }
             case 'l': {
-                is_login = 1;
+                is_login = true;
                 break;
             }
             case 'n': {