nmtui: fix deletion confirmation with ESC key press

When deleting a profile, the confirmation dialog shows "Cancel" and
"Delete" buttons. ESC key should do nothing, but in some distributions
like Debian and Ubuntu newt has a downstream patch that enables it (see
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=584098).

In that case, when pressing ESC the return value of the dialog is not
"Cancel" (1) or "Delete" (2), but the "otherwise" value (0). Fix it by
not checking if "Cancel" is pressed. Instead, check if "Delete" was
pressed, and continue deleting only in that case.

Also, fix the doc comment that incorrectly says that the dialog returns
0/1 for the buttons, it is 1/2.

src/libnmt-newt/nmt-newt-utils.c

@@ -282,7 +282,7 @@ nmt_newt_message_dialog(const char *message, ...)
  * Displays the given message in a dialog box with two buttons with
  * the indicated labels, and waits for the user to click one.
  *
- * Returns: which button was clicked: 0 for @button1 or 1 for @button2
+ * Returns: which button was clicked: 1 for @button1, 2 for @button2 or 0 otherwise
  */
 int
 nmt_newt_choice_dialog(const char *button1, const char *button2, const char *message, ...)

src/nmtui/nmtui-edit.c

@@ -511,7 +511,7 @@ nmt_remove_connection(NMRemoteConnection *connection)
                                     _("Delete"),
                                     _("Are you sure you want to delete the connection '%s'?"),
                                     nm_connection_get_id(NM_CONNECTION(connection)));
-    if (choice == 1)
+    if (choice != 2)
         return;
 
     g_object_ref(connection);